Stats without Tears
10. Hypothesis Tests

Summary: You want to know if something is going on (if there’s some effect). You assume nothing is going on (null hypothesis), and you take a sample. You find the probability of getting your sample if nothing is going on (p-value). If that’s too unlikely, you conclude that something is going on (reject the null hypothesis). If it’s not that unlikely, you can’t reach a conclusion (fail to reject the null).

10A.  Testing a Proportion (Binomial Data)

Remember the Swain v. Alabama example? In a county that was 26% African American, Mr. Swain’s jury pool of 100 men had only eight African Americans. In that example, you assumed that selection was not racially biased, and on that basis you computed the probability of getting such a low proportion. You found that it was very unlikely. This disconnect between the data and the claim led you to reject the claim.

You didn’t know it, but you were doing a hypothesis test. This is the standard way to test a claim in statistics: assume nothing is going on, compute the probability of getting your sample, and then draw a conclusion based on that probability. In this chapter, you’ll learn some formal methods for doing that.

The basic procedure of a hypothesis test or significance test is due to Jerzy Neyman (1894–1981), a Polish American, and Egon Pearson (1895–1980), an Englishman. They published the relevant paper in 1933.

We’re going to take a seven-step approach to hypothesis tests. The first examples will be for binomial data, testing a claim about a population proportion. Later in this chapter you’ll use a similar approach with numeric data to test a claim about a population mean. In later chapters you’ll learn to test other kinds of claims, but all of them will just be variations on this theme.

10A1.  Example 1: Swain v. Alabama

Step 1: Hypotheses

Your first task is to turn the claim into algebra. The claim may be that nothing is going on, or that something is going on. You always have two statements, called the null and alternative hypotheses.

Definition: The null hypothesis, symbol H₀, is the statement that nothing is going on, that there is no effect, “nothin’ to see here. Move along, folks!” It is an equation, saying that p, the proportion in the population (which you don’t know), equals some number.

Definition: The alternative hypothesis, symbol H₁, is the statement that something is going on, that there is an effect. It is an inequality, saying that p is different from the number mentioned in H₀. (H₁ could specify <, >, or just ≠.)

The hypotheses are statements about the population, not about your sample. You never use sample data in your hypotheses. (In real life you can’t make that mistake, since you write your hypotheses before you gather data. But in the textbook and the classroom, you always have sample data up front, so don’t make a rookie mistake.)

You must have the algebra (symbols) in your hypotheses, but it can also be helpful to have some English explaining the ultimate meaning of each hypothesis, or the consequences if each hypothesis is true. Here you want to know whether there’s racial bias in jury selection in the county.

You don’t want to know if the proportion of African Americans in Mr. Swain’s jury pool is less than 26%: obviously it is. You want to know if it’s too different — if the difference is too great to be believable as the result of random chance.

Write your hypotheses this way:

Obviously those can’t both be true. How will you choose between them? You’ll compute the probability of getting your sample (or a more unexpected one), assuming that the null hypothesis H₀ is true, and one of two things will happen. Maybe the probability will be low. In that case you rule out the possibility that random chance is all that’s happening in jury selection, and you conclude that the alternative hypothesis H₁ is true. Or maybe the probability won’t be too low, and you’ll conclude that this sample isn’t unusual (unexpected, surprising) for the claimed population.

The number in your null hypothesis H₀, with binomial data, is called p_o because it’s the proportion as given in H₀. (You may want to refer to the Statistics Symbol Sheetto help you keep the symbols straight.)

Step 2: Significance Level

Okay, you’re looking to figure out if this sample is inconsistent with the null hypothesis. In other words, is it too unlikely, if the null hypothesis H₀ is true? But what do you mean by “too unlikely”? Back in Chapter 5, we talked about unusual events, with a threshold of 5% or 0.05 for such events. We’ll use that idea in hypothesis testing and call it a significance level.

Definition: The significance level, symbol α (the Greek letter alpha), is the chance of being wrong that you can live with. By convention, you write it as a decimal, not a percentage.

A significance level of 0.05 is standard in business and science. If you can’t tolerate a 5% chance of being wrong — if the consequences are particularly serious — use a lower significance level, 0.01 or 0.001 for example. (0.001 is common if there’s a possibility of death or serious disease or injury.) If the consequences of being wrong are especially minor, you might use a higher significance level, such as 0.10, but this is rare in practice.

In a classroom setting, you’re usually given a significance level α to use.

Later in this chapter, you’ll see that the significance level α is actually concerned with a particular way of being wrong, a Type I error.

Step RC: Requirements Check

Back in Chapter 8, you learned the CLT’s requirements for binomial data: random sample not larger than 10% of population, and at least 10 successes and 10 failures expected if the null hypothesis is true. You compute expected successes as np_o by using p_o, which is the number from H₀. Expected failures are then sample size minus expected successes, n−np_o in symbols. Steps 3 and 4 need the sampling distribution of the proportion to be a ND, so you must check the requirements as part of your hypothesis test.

You might wonder about the first test. “The county may say it’s random, but I don’t believe it. Isn’t that why we’re running this test?” Good question! Answer: Every hypothesis test assumes the null hypothesis is true and computes everything based on that. If you end up deciding that the sample was too unlikely, in effect you’ll be saying “I assumed nothing was going on, but the sample makes that just too hard to believe.”

This same idea — the null hypothesis H₀ is innocent till proven guilty — explains why you use 0.26 (p_o) to figure expected successes and failures, not 0.08 (p̂). Again, the county claims that there’s no racial bias. If that’s true, if there’s no funny business going on, then in the long run 26% of members of jury pools should be African American.

Comment: Usually, if requirements aren’t met you just have to give up. But for one-population binomial data, where the other two requirements are met but expected successes or failures are much under 10, you can use MATH200A part 3 to compute the p-value directly. There’s an example in “Small Samples”, below.

Steps 3/4: Test Statistic and p-Value

This is the heart of a hypothesis test. You assume that the null hypothesis is true, and then use what you know about the sampling distribution to ask: How likely is this sample, given that null hypothesis?

Definition: A test statistic is a standardized measure of the discrepancy between your null hypothesis H₀ and your sample. It is the number of standard errors that the sample lies above or below H₀.

You can think of a test statistic as a measure of unbelievability, of disagreement between H₀ and your sample. A sample hardly ever matches your null hypothesis perfectly, but the closer the test statistic is to zero the better the agreement, and the further the test statistic is from 0 the worse the sample and the null hypothesis disagree with each other.

Because you showed that the sampling distribution is normal and the standard error of the proportion is implicitly known, this is a z test. The test statistic is z = (p̂−p_o) / σ_p̂  where  , but as you’ll see your calculator computes everything for you.

Definition: The p-value is the probability of getting your sample, or a sample even further from H₀, if H₀ is true. The smaller the p-value, the stronger the evidence against the null hypothesis.

Inferential Statistics: Basic Cases tells you that binomial data in one population are Case 2. This is a hypothesis test of population proportion, and you use 1-PropZTest on your calculator.

To get to that menu selection, press [STAT] [◄] [5]. Enter p_o from the null hypothesis H₀, followed by the number of successes x, the sample size n, and the alternative hypothesis H₁. Write everything down before you select Calculate. When you get to the output screen, check that your alternative hypothesis H₁ is shown correctly at the top of the screen, and then write down everything that’s new.

By convention, we round the test statistic to two decimal places and the p-value to four decimal places.

When the p-value is less than one in ten thousand, you need more than four decimal places. Some authors just write “p <.0001” when the p-value is that small; they figure nobody cares about fine shades of very low probability. Feel free to use that alternative.

Caution! Watch for powers of 10 (E minus whatever) and never write something daft like “p-value = 2.0346”.

What do these outputs of the 1-PropZTest tell you? The sample proportion, p̂ = 0.08, is more than 4 standard errors below the supposed population proportion, p_o = 0.26. Your test statistic is z = −4.10. Since 95% of samples have z-scores within ±2, this is surprising. How surprising, that’s what the p-value tells you.

How likely is it to get this sample, or one with even a smaller sample proportion, if the null hypothesis H₀ is true? The p-value is 0.000 020, so if there’s no racial bias in selection then there are only two chances in a hundred thousand of getting eight or fewer African Americans in a 100-man jury pool. (There’s a lot more about interpreting the p-value later in this chapter.)

You don’t actually use the z-score, but I want you to understand something about what a test statistic is. Every case you study will have a different test statistic, and in fact choosing a test statistic is the main difference between cases.

Why does one step have two numbers? In the olden days, when dinosaurs roamed the earth and a slide rule was the hot new thing, you had to compute the SEP and then the z-score; that was step 3. Then you had to look up z in a printed table to find the p-value; that was step 4. The TI-83 or TI-84 gives you both at the same time, but I’ve kept the numbering of steps.

Step 5: Decision Rule

There are two and only two possibilities, and all you have to do is pick the correct one based on your p-value and your α:

p < α. Reject H₀ and accept H₁.

or

p > α. Fail to reject H₀.

Caution! There are lots of p’s in problems involving population proportions (Case 2), so make sure you select the right one. The p-value is the first p on the 1-PropZInt output screen.

You can add the numbers, if you like — p < α (0.000 020 < 0.05) — but the symbols are required.

What are you saying here? The p-value was very small, so that means the chance of getting this sample, if there’s no racial bias, was very small. Previously, you set a significance level of 0.05, meaning you would consider this sample too unlikely if its probability was under 5%. Its probability is under 5%, so the sample and the null hypothesis contradict each other. The sample is what it is, so you can’t reject the sample. Therefore you reject H₀ and accept H₁ — you declare that there is racial bias.

Another way to look at it: Any sample will vary from the population because random selection is always operating to produce sampling error. But the difference between this sample and the supposed population proportion is just too great to be produced by random selection alone. Something else must be going on also. That something else is the alternative hypothesis H₁.

Definition: When the p-value is below α, the sample is too unlikely to come from ordinary sample variability alone, and you have a significant result, or your result is statistically significant.

You always select a significance level before you know the p-value. If you could first get the p-value and then specify a significance level, you could get whichever result you wanted, and there would be no point to doing a hypothesis test at all. Choosing α up front keeps you honest.

Step 6: Conclusion (in English)

Since you accepted H₁ in the previous step, that’s your conclusion. If you have already written it in English as part of the hypotheses, as I did, then most of your work is already done. You do need to add the significance level or the p-value, so your conclusion will look something like one of these:

or

If you’re publishing your hypothesis test, you’ll want to write a thorough conclusion that still makes sense if it’s read on its own. But in class exercises you don’t have to write so much. It’s enough to write “At the 0.05 significance level, there is racial bias in jury selection” or “There is racial bias in jury selection (p = 0.000 020)”.

10A2.  Example 2: Cancer Screening

The Colorectal Cancer Screening Guidelines (CDC 2014 [see “Sources Used” at end of book]) recommend a colonoscopy every ten years for adults aged 50 to 75. A public-health researcher believes that only a minority are following this recommendation. She interviews a simple random sample of 500 adults aged 50–75 in Metropolis (pop. 6.4 million) and finds that 235 of them have had a colonoscopy in the past ten years. At the 0.05 level of significance, is her belief correct?

Solution: The population is adults aged 50–75 in Metropolis. You want to know whether a minority of them — under 50% — follow the colonoscopy guideline. Each person either does or does not, so you have binomial data, a test of proportion (Case 2 in Inferential Statistics: Basic Cases). Try to write out the hypothesis test yourself before you look at mine below.

Reminder: Even though you already have the sample data in the problem, when you write the hypotheses, ignore the sample. In principle, you write the hypotheses, then plan the study and gather data. If you use any of the sample data in the hypotheses, something is wrong.

You should have written something pretty close to this:

Important: When p is greater than α, you fail to reach a conclusion. In this situation, you must use neutral language. You mention both possibilities without giving more weight to either one, and you use words like “impossible to say” or “can’t determine”.

This is unsatisfying, frankly. You go through all the trouble of gathering data and then you end up with a non-conclusion. Can anything be salvaged from this mess?

Yes, you can do a confidence interval. This at least will let you set bounds on what percent of all seniors follow the guidelines. You’ve already tested requirements as part of the hypothesis test, so go right into your calculations and conclusion. You’re free to pick any confidence level you wish, but 95% is most usual.

1-PropZInt, 235, 500, .95

outputs: (.42625, .51375)

42.6% to 51.4% of Metropolis seniors aged 50–75 follow the CDC guideline on screening for colorectal cancer.

In a classroom setting, or on regular homework, if you’re assigned a hypothesis test do that and don’t feel obligated to do a confidence interval also. But in real life, and on labs and projects for class, you’ll usually want to do both.

10A3.  Example 3: Small Samples

What if your sample is so small that expected successes np_o or expected failures n−np_o are under 10? You can no longer use 1-PropZTest, which assumes that the sampling distribution of the proportion is ND, but you can compute the binomial probability directly as long as the other two requirements are still met (SRS and 10n≤N). Only the calculation of the p-value changes.

Example: In 2001, 9.6% of Fictional County motorists said that fuel efficiency was the most important factor in their choice of a car. For her statistics project, Amber set out to prove that the percentage has increased since then. She interviewed 80 motorists in a systematic sample of those registering vehicles at the DMV, and 13 of them said that fuel efficiency was the most important factor in their choice of a car. Test her hypothesis, at the 0.05 significance level.

Please write out your hypothesis test before you look at mine.

10B.  Sharp Points

Hypothesis tests are based on a simple idea, but there are lots of details to think about. This section clarifies some important ideas about the philosophy and practice of a hypothesis test.

10B1.  Type I and Type II Errors

Definition: A Type I error is rejecting the null hypothesis when it’s actually true.

Definition: A Type II error is failing to reject the null hypothesis when it’s actually false.

A Type I error usually causes you to do something you shouldn’t; a Type II error usually represents a missed opportunity.

Example 4: Suppose your alternative hypothesis H₁ is that a new headache remedy PainX helps a greater proportion of people than aspirin.

A Type I error — rejecting H₀ and accepting H₁ when H₀ is actually true — would have you announce that PainX helps more people when in fact it doesn’t. People would then buy PainX instead of aspirin, and their headache would less likely be cured. This is a bad thing.

On the other hand, a Type II error — failing to reject H₀ when it’s actually false — would mean you announce an inconclusive result. This keeps PainX off the market when it actually would have helped more people than aspirin. This too is a bad thing.

Example 5: You’re on a jury, and you have to decide whether the accused actually committed the murder. What would be Type I and Type II errors?

To answer that you need to identify your null hypothesis H₀. Remember that it’s always some form of “nothing going on here.” In this case, H₀ would be that the defendant didn’t commit the murder, and H₁ would be that he did.

A Type I error would be condemning an innocent man; a Type II error would be letting a guilty man go free. In our legal system, a defendant is not supposed to be found guilty if there is a reasonable doubt; this would correspond to your α. Probably α = 0.05 is not good enough in a serious case like murder, where a Type I error would mean long jail time or execution, so if you’re on a jury you’d want to be more sure than that.

“Okay then,” you say, “I’ll have to be super careful and not make mistakes.” But remember from Chapter 1: In statistics, “errors” aren’t necessarily mistakes. Errors are discrepancies between your results and reality, whatever their cause. Type I and Type II errors are not mistakes in procedure.

Even if you do everything right in your hypothesis test, you can’t be certain of your answer, because you can never get away from sample variability.

How often will these errors occur? This is where your significance level comes into play. If you perform a lot of tests at α = 0.05, then in the long run a Type I error will occur one time in twenty. It’s too big for these pages, but there’s a cartoon at xkcd.com that illustrates this perfectly. The probability of a Type II error has the symbol β (Greek letter beta) and it has to do with the “power” of the test, its ability to find an effect when there’s an effect to be found. β belongs to a more advanced course, and I don’t do anything with it in this book.

Earlier, I said that your significance level α is the chance of being wrong that you can live with. Now I can be a little more precise. α is not the chance of any error; α is the chance of a Type I error that you can live with. If one Type I error in 20 hypothesis tests is unacceptable, use a lower significance level — but then you make a Type II error more likely. If that’s unacceptable, increase your sample size.

Somebody is making a mint off the following chart. It’s in every stats textbook I’ve seen, so you may as well have it too:

10B2.  One-Tailed or Two-Tailed?

Pick the Right Hypotheses

There are two main situations where a one-tailed test makes sense: “(a) where there is truly concern for the outcomes in one [direction] only and (b) where it is completely inconceivable that the results could go in the opposite direction.”

—Dubey, quoted by Kuzma and Bohnenblust (2005, 132) [see “Sources Used” at end of book]

With a one-tailed test, say for μ<4.5, you’re saying that you consider “equal to 4.5” and “greater than 4.5” the same thing, that if μ isn’t less than 4.5 then you don’t care whether it’s equal or it’s greater. Sometimes you really don’t care, but very often you do. If the problem statement is ambiguous, or if this is real life and you have to do a hypothesis test, how do you decide whether to do a one-tailed or two-tailed test?

Testing two-tailed doesn’t prejudge a situation. Do a two-tailed test unless you can honestly say, without looking at the data, that only one direction of difference matters, or only one direction is possible.

Example 6: An existing drug cures people in an average of 4.5 days, and you’re testing a new drug. If you test for μ<4.5, you’re saying that it doesn’t matter whether the new drug takes the same time or takes more time. But that’s wrong: it matters very much. You want to test whether the new drug is different (μ≠4.5). Then if it’s different, you can conclude whether it’s faster or slower.

Another way to look at this whole business: a one-tailed test essentially doubles your α — you’re much more likely to reach a conclusion with dicey data. But that means double the risk of being wrong with a Type I error — not a good thing!

Sometimes the same situation can call for a different test, depending on your viewpoint.

Example 7: You’re the county inspector of weights and measures, checking up on a dairy and its half gallons of milk. Legally, half a gallon is 64 fluid ounces. To a government inspector, “Dairylea gives 64.0 ounces in the average half gallon” and “Dairylea gives more than 64.0 ounces in the average half gallon” are the same (legal), and you care only about whether Dairylea gives less (illegal). A one-tailed test (<) is correct.

But now shift your perspective. You’re Dairylea management. You don’t want to short the customers because that’s illegal, but you don’t want to give too much because that’s giving away money. You make a two-tailed test (≠).

p < α in Two-Tailed Test: What Does it Tell You?

After a two-tailed test, if p<α then you can interpret the result as one-tailed.

Example 8: You want to test whether your candidate’s approval rating has changed from the previous dismal 40% after a major policy announcement. Your H₁ is p ≠ 0.4, and 170 out of a random sample of 500 voters approve (p̂ = 34%). Your p-value is 0.0062, so you reject H₀ and accept H₁. You conclude that the candidate’s approval rating has changed.

But you can go further and say that her approval rating has dropped. You do this by combining the facts that (a) you’ve proved that approval rating is different, which means it must be either less or more than 40%, and (b) the sample p̂ was less than p_o (40%).

You can phrase your conclusion something like this, first answering the original question then going beyond it: The candidate’s approval rating has changed from 40% after the speech (p = 0.0062). In fact, it has dropped.

Your justification is the relationship between Confidence Interval and Hypothesis Test (later in this chapter), but you don’t actually have to compute the CI. When p < α in a two-tailed test, p_o is outside the confidence interval (at the matching confidence level).

• When p̂ is above p_o and the p-value is < α, the whole CI (if you computed it) would be above p_o so you know the true proportion is greater than p_o.
• Conversely, if p̂ is below p_o and the p-value is < α, the whole CI would be below p_o so you know the true proportion is below p_o.

10B3.  What Does the p-Value Mean?

Summary: The p-value tells you how likely it is to get the sample you got (or a more extreme sample) if the null hypothesis is true.

Many people are confused about the p-value. They try to read too much into it, or they try to simplify it.

Part of the problem is trying to fit the meaning into the traditional structure of a one-sentence definition, so let’s try a story instead. In your experiment, you got a certain result, a sample mean or sample proportion. Assume that the null hypothesis is true. If H₀ is true, the properties of the sampling distribution tell you how likely it is to get this sample result, or one even further away from H₀. That likelihood is called the p-value.

The one-tailed p-value is exactly the probability that you computed with normalcdf in Chapter 8. When that’s less than 0.5, the two-tailed p-value is exactly double the one-tailed p-value.

If the p-value is small, your results are in conflict with H₀, so you reject the null and accept the alternative. If the p-value is larger, your sample is not in conflict with H₀ and you fail to reject the null, which is stats-talk for failing to reach any kind of conclusion.

In a nice phrase, Sterne and Smith [see “Sources Used” at end of book] say that p-values “measure the strength of the evidence against the null hypothesis; the smaller the p-value, the stronger the evidence against the null hypothesis.” They also quote R. A. Fisher on interpreting a p-value: “If P is between 0.1 and 0.9 there is certainly no reason to suspect the hypothesis tested. If it is below 0.02 it is strongly indicated that the hypothesis fails to account for the whole of the facts. We shall not often be astray if we draw a conventional line at 0.05.”

The message here is that p-values fall on a continuum; you can’t just arbitrarily divide them into “significant” and “not significant” once and for all.

The p-value is the likelihood, if H₀ is actually true, that random chance could give you the results you got, or results even further from H₀. It is a conditional probability:

p-value = P(this sample given that H₀ is true)

Yes, that seems convoluted — because it is. Alas, there just isn’t any description of a p-value that is both correct and simple.

The p-value is not the probability that either hypothesis is true or false:

• The p-value is not the probability that H₀ is true.
• The p-value is not the probability that H₀ is false.
• The p-value is not the probability that H₁ is true.
• The p-value is not the probability that H₁ is false.
• The p-value is not the probability that your results are due to random chance.
• The p-value is not the probability that your results are not due to random chance.

The p-value is not any of the above because they are all plain probabilities. Once again, the p-value is just a measure of how likely your results would be if  H₀ is true and random chance is the only factor in selecting the sample.

The p-value tells you how unlikely this sample (or a more extreme one) is if the null hypothesis is true. The more unlikely (surprising, unexpected), the lower the p-value, and the more confident you can feel about rejecting H₀.

There’s one other thing: the p-value is not a measure of the size or importance of an effect. That gets into statistical significance versus practical significance.

10B4.  Practical and Statistical Significance

If your p-value is less than your significance level α, your result is statistically significant. That low p-value, Wheelan (2013, 11) [see “Sources Used” at end of book] writes, means that your result is “not likely to be the product of chance alone”. That’s all that statistical significance means. But even if a result is statistically significant, it may not be practically significant.

Example 9: Suppose that your p-value for “PainX is more likely to help a person than aspirin” is 0.000 002. You’re pretty darn sure that PainX is better. But to determine whether the result is practically significant, you have to ask not just whether PainX is better, but by how much.

One way to evaluate practical significance is to compute a confidence interval about the effect size. In this case, the 95% confidence interval is that a person is between 1.14 and 2.86 percentage points more likely to be helped by PainX than aspirin. Oh yes, and aspirin costs a buck for 100 tablets, where PainX costs $29.50 for ten. Most people would say this result has no practical significance. They’re not going to plunk down $30 for a few pills that are only 2% more likely to help them than aspirin.

How can you get such a low p-value when the size of the effect is small? The answer is in extremely large sample sizes. In this made-up case, PainX helped 15,500 people in a sample of 25,000, and aspirin helped 15,000 in a sample of 25,000. When you have really large samples, be especially alert to the issue of statistical versus practical significance.

10B5.  Conclusions: Write ’em Right!

used by permission; source: https://xkcd.com/892/ (accessed 2021-11-15)

When p < α, you reject H₀ and accept H₁.

If your p-value is less than your significance level, you have shown that your sample results were unlikely to arise by chance if H₀ is true. The data are statistically significant. You therefore reject H₀ and accept H₁.

Details: Assuming that H₀ is true, the sample you got is surprising (unexpected, unusual). The data are inconsistent with the null hypothesis — they can’t both be true. The data are what they are, and if the sample was properly taken you have to believe in it. Therefore, H₀ is most likely false. If H₀ is false, its opposite H₁ is true.

You accept H₁, but you haven’t proved it to a certainty. There’s always that p-value chance that the sample results could have occurred when H₀ is true. That’s why you say you “accept” H₁, not that you have “proved” H₁.

Compare to a jury verdict of “guilty”. It means the jury is convinced that the probability (p) that the defendant is innocent is less than a reasonable doubt (significance level, α). It doesn’t mean there is no chance he’s innocent, just that there is very little chance.

Don’t say the average package “might” be less than the stated weight or “appears to be” less than the stated weight. When you reject H₀, state the alternative as a fact within the stated significance level, or preferably with the p-value. (Again, compare to a jury verdict. The jury doesn’t say the defendant “might be guilty”.)

See also: Take published conclusions with a grain of salt. Even professional researchers can misuse hypothesis tests. “Data mining” (first gathering data, then looking for relationships) is one problem, but not the only one. See Why Most Published Research Findings Are False (Ioannidis 2005 [see “Sources Used” at end of book]). If you find the article heavy going, just scroll down to read the example in Box 1 and then the corollaries that follow.

When p > α, you fail to reject H₀.

If your p-value is greater than your significance level, you have shown that random chance could account for your results if H₀ is true. You don’t know that random chance is the explanation, just that it’s a possible explanation. The data are not statistically significant.

You therefore fail to reject H₀ (and don’t mention H₁ in step 5). The sample you have could have come about by random selection if H₀ is true, but it could also have come about by random selection if H₀ is false. In other words, you don’t know whether H₀ is actually true, or it’s false but the sample data just happened to fall not too far from H₀.

Compare to a jury verdict of “not guilty”. That could mean the defendant is actually innocent, or that the defendant is actually guilty but the prosecutor didn’t make a strong enough case.

Example 11: Suppose your null hypothesis is “the average package contains the stated net weight,” your alternative is “the average package contains less than the stated net weight,” and your significance level α is 0.05.

If you compute a p-value of 0.0788, which is > α, you fail to reject H₀ in step 5, but how do you state your conclusion in step 6?

There are two kinds of answer, depending on who you talk to. Some people say “there’s insufficient evidence to prove that the average package is underweight”; others say “we can’t tell whether the average package is underweight or not.” Of course there are many ways to write a conclusion in English, but ultimately they boil down to “we can’t prove H₁” (or the equivalent “we can’t disprove H₀”) versus “we can’t reach a conclusion either way.”

Does it matter? Yes, I think it does.

• “We can’t prove H₁” is true, but it’s only part of the truth. The whole truth is that we can’t prove or disprove H₀ when the p-value is greater than the significance level.
• Non-technical people are intimidated by statistics and may not realize what is meant. Tell them “we can’t disprove A” and they may think A is true. And if you say “we can’t prove B”, people may think B is false. If you say straightforwardly, “we can’t determine which one is true”, then there’s no risk of people jumping to a false conclusion.
• As a practical matter, we don’t do hypothesis testing just for the fun of it. We want to know something about the real world, not just out of sheer curiosity but because the result will determine what we do. Starting to market a new drug costs mucho dinero, so “we can’t prove that the new drug is better” probably means the drug won’t go to market, and that would be too bad if the new drug actually is better. “We can’t determine whether the new drug is better or not” essentially says “a new study is needed, probably with larger samples.”

Please understand: It’s not that the people writing the conclusions are confused (well, usually not). The problem is confusion among people reading the conclusions.

Advice: It’s the same advice I’ve given before: Tailor your presentation to your audience. If you’re presenting to technical people, the one-sided forms are okay, and you could answer Example 11 with something like “there’s insufficient evidence, at the 0.05 significance level, to show that the average package is under weight” or “… to reject the hypothesis that the average package contains the stated net weight.” (Since the p-value gives more information, you could give that instead of the significance level.)

But if your audience is non-technical people, don’t expect them to understand a two-sided truth from a one-sided conclusion. Instead, use neutral language, such as “We can’t determine from the data whether the average package is underweight or not (p = 0.0788).” (You could state the significance level instead of the p-value.)

What if the p-value is very large?

If your p-value is very large, say bigger than 0.5, there’s a good chance you’ve made a mistake. Check carefully whether you should be testing <, ≠, or >. Also check whether you’re testing against the wrong number. For instance, suppose your H1 is that a coin comes up heads more than a third of the time. A few dozen flips will probably yield a p-value very close to 1. This is the statistical equivalent of “Well, duh!”

Sometimes large p-values are correct, but those situations are rare enough that you should be suspicious.

Can we never accept the null hypothesis?

Not as a matter of strict logic, no. But there are circumstances where the data do suggest that the null hypothesis is true. The most important of these is when multiple experiments fail to reject H₀. Here’s why.

Suppose you do an experiment at the 0.05 significance level, and your p-value is greater than that. Maybe H₀ is really true; maybe it’s false but this particular sample happened to be close to H₀. You can’t tell — you’ve failed to disprove H₀ but that doesn’t mean it’s necessarily true.

But suppose other experimenters also get p-values > 0.05. They can’t all be unlucky in their samples, can they?

If you keep giving the universe opportunities to send you data that contradict the null hypothesis, but you keep getting data that are consistent with the null, then you begin to think that the null hypothesis shouldn’t be rejected, that it’s actually true.

This is why scientists always replicate experiments. If the first experiment fails to reject H₀, they don’t know whether H₀ is true or they were just unlucky in their sample. But if several experiments fail to reject the null — always assuming the experiments are properly conducted — then they begin to have confidence in the theory.

What if an experiment does reject H₀? Is that it, game over? Not necessarily. Remember that even a true H₀ will get rejected one time in 20 when tested at the 0.05 level. Once again, the answer is replication. If they get more “reject H₀”,scientists know that the first one wasn’t just a statistical fluke. But if they get a string of “fail to reject H₀”, then it’s likely that the first one was just that one in 20, and H₀ is actually true.

10C.  Testing a Mean (Numeric Data)

Summary: Just as you used a TInterval in Chapter 9 to make a confidence interval about μ for numeric data, you use a T-Test to perform the hypothesis test.

Typically you don’t know σ, the standard deviation (SD) of the population, and therefore you don’t know the standard error σ/√n either. So you estimate the standard error as s/√n, using the known SD of the sample. That means that the test statistic is:

t = (x̅−μ_o) / (s/√n)

The t statistic is the estimated number of standard errors between your sample mean and the hypothetical population mean.

You met the t distribution when you computed confidence intervals in Chapter 9. Compared to z, the t distribution is a little flatter and more spread out, especially for small samples, so p-values tend to be larger.

Let’s jump in and do a t test. The numbered steps are almost the same as they were in the examples with binomial data — you just have the necessary variations for working with numeric data. Because I’ll be adding some commentary, I’ve put boxes around what I expect to see from you for a problem like this. (Refer to Seven Steps of Hypothesis Tests if you don’t know the steps very well yet.)

It hardly ever happens, but if you do know the SD of the population you can do a z test instead of a t test. Since the z distribution is a bit less spread out than the t distribution, for very small samples the p-values are typically a bit lower with a z test than with a t. But the difference is rarely enough to change the result — and again, you are quite unlikely to know the SD of the population, so a z test is quite unlikely to be the right one.

10C1.  Example 12: Bank Deposits

The management claims that the average cash deposit is $200.00, and you’ve taken a random sample to test that:

 192.68 188.24 152.37 211.73 201.57   167.79 177.19 191.15 209.22 178.49 
 185.90 226.31 192.38 190.23 156.13   224.07 191.78 203.45 186.40 160.83 

At the 0.05 significance level, does this sample show that the average of all cash deposits is different from $200?

Solution: The data type is numeric, and the population SD σ is unknown, so this is a test of a population mean, Case 1 from Inferential Statistics: Basic Cases. Your hypotheses are:

Comment: Even though you already have the sample data in the problem, when you write the hypotheses, ignore the sample. In principle, you write the hypotheses, then plan the study and gather data. If you use any of the sample data in the hypotheses, something is wrong.

So you don’t use numbers from the sample in your hypotheses, and you don’t use the sample to help you decide whether the alternative hypothesis H₁ should have < ≠, or >.

The significance level was given in the problem. (Problems will usually give you an α to use.)

Next is the requirements check. Even though it doesn’t have a number, it’s always necessary. In this case, n = 20, which is less than 30, so you have to test for normality and verify that there are no outliers.

Enter your data in any statistics list (I used L5), and check your data entry carefully. Use the MATH200A program “Normality chk” to check for a normal distribution and “Box-whisker” to verify that there are no outliers.

You don’t need to draw the plots, but do write down r and crit and show the comparison, and do check for outliers. (For what to do if you have outliers, see Chapter 3.)

Now it’s time to compute the test statistic (t) and the p-value.

On the T-Test screen, you have to choose Data or Stats just as you did on the TInterval screen. You have the actual data, so you select Data on the T-Test screen, instead of Stats. Then the sample mean, sample SD, and sample size are shown on the output screen, so you write them down as part of your results. Always write down x̅, s, and n.

The decision rule is the same for every single hypothesis test, regardless of data type. In this case:

And as usual, you can write your conclusion with the significance level or the p-value:

Or,

Remember what happens when you do a two-tailed test (≠ in H₁) and p turns out less than α: After you write your “different from” conclusion, you can go on to interpret the direction of the difference. See p < α in Two-Tailed Test.

In a classroom exercise, if you were asked to do a hypothesis test you would do a hypothesis test and only a hypothesis test. But in real life, and in the big labs for class, it makes sense to answer the obvious question: If the true mean is less than $200.00, what is it?

You don’t have to check requirements for the CI, because you already checked them for the HT.

10C2.  Example 13: Smokers and Retirement

Here’s an example where you have statistics without the raw data. It’s adapted from Sullivan (2011, 483) [see “Sources Used” at end of book].

According to the Centers for Disease Control, the mean number of cigarettes smoked per day by individuals who are daily smokers is 18.1. Do retired adults who are daily smokers smoke less than the general population of daily smokers?

To answer this question, Sascha obtains a random sample of 40 retired adults who are current daily smokers and record the number of cigarettes smoked on a randomly selected day. The data result in a sample mean of 16.8 cigarettes and a SD of 4.7 cigarettes.

Is there sufficient evidence at the α = 0.01 level of significance to conclude that retired adults who are daily smokers smoke less than the general population of daily smokers?

Solution: Start with the hypotheses. You’re comparing the unknown mean μ for retired smokers to the fixed number 18.1, the known mean for smokers in general. Since the data type is numeric (number of cigarettes smoked), and there’s one population, and you don’t know the SD of the population, this is Case 1, test of population mean, from Inferential Statistics: Basic Cases.

(1)	H0: μ = 18.1, retired smokers smoke the same amount as smokers in general H1: μ < 18.1, retired smokers smoke less than smokers in general Comment: The claim is a population mean of 18.1, so you use 18.1 in your hypotheses. Using the sample mean of 16.8 in Step 1 is a rookie mistake, one of the Top 10 Mistakes of Hypothesis Tests. Never use sample data in your hypotheses. Comment: Why does H1 have < instead of ≠? The short answer is: that’s what the problem says to do. In the real world, you would do a two-tailed test (≠) unless there’s a specific reason to do a one-tailed test (< or >); see One-Tailed or Two-Tailed? (earlier in this document). Presumably there’s some reason why they are interested only in the case “retired smokers smoke less” and not in the case “retired smokers smoke more”.
(2)	α = 0.01
(RC)	Random sample (given). n > 30. 10n = 10×40 = 400, less than the total number of retired smokers. Therefore the sampling distribution is normal.
(3/4)	T-Test: μo=18.1, x̅=16.8, s=4.7, n=40, μ<μo outputs: t=−1.75, p=0.0440
(5)	p > α. Fail to reject H0.
(6)	At the 0.01 level of significance, we can’t determine whether the average number of cigarettes smoked per day by retired adults who are current smokers is less than the average for all daily smokers or not. Or, We can’t tell whether the average number of cigarettes smoked per day by retired adults who are current smokers is less than the average for all daily smokers or not (p = 0.0440).

When you fail to reject H₀, you cannot reach any conclusion. You must use neutral language in your non-conclusions. Please review When p > α, you fail to reject H₀ earlier in this chapter.

10D.  Confidence Interval and Hypothesis Test

Example 14: The baseline rate for heart attacks in diabetes patients is 20.2% in seven years. You have a new diabetes drug, Effluvium, that is effective in treating diabetes. Clinical trials on 89 patients found that 27 (30.3%) had heart attacks. The 95% confidence interval is 20.8% to 39.9% likelihood of heart attack within seven years for diabetes patients taking Effluvium. What does this tell you about the safety of Effluvium?

Solution: Okay, you’re 95% confident that Effluvium takers have a 20.8% to 39.9% chance of a heart attack within seven years. If you’re 95% confident that their chance of heart attack is inside that interval, then there’s only a 5% or 0.05 probability that their chance of heart attack is outside the interval, namely <20.8% or >39.9%.

But 20.2% is outside the interval, so there’s less than a 0.05 chance that the true probability of heart attack with Effluvium is 20.2%.

CI and HT calculations both rely on the sampling distribution. The open curve centered on 20.2% shows the sampling distribution for a hypothetical population proportion of 20.2%. Only a very small part of it extends beyond 30.3%, the proportion of heart attacks you actually found in your sample.

The chance of getting your sample, given a hypothetical proportion p_o in the population, is the p-value. If p_o = 20.2%, your sample with p̂ = 30.3% would be unlikely (p-value below 0.05). You would reject the null hypothesis and conclude that Effluvium takers have a different likelihood of heart attack from other diabetes patients, at the 0.05 significance level. Further, the entire confidence interval is above the baseline value, so you know that Effluvium increases the likelihood of heart attack in diabetes patients.

At significance level 0.05, a two-tailed test against any value outside the 95% confidence interval (the shaded curve) would lead to rejecting the null hypothesis. And you can say the same thing for any other significance level α and confidence level 1−α.

What if the interval does include the baseline or hypothetical value? Then you fail to reject the null hypothesis.

Example 15: A machine is supposed to be turning out something with a mean value of 100.00 and SD of 6.00, and you take a random sample of 36 objects produced by the machine. If your sample mean is 98.4 and SD is 5.9, your 95% confidence interval is 96.4 to 100.4.

Now, can you make any conclusion about whether the machine is working properly?

Solution: Well, you’re 95% confident that the machine’s true mean output is somewhere between 96.4 and 100.4. With this sample, you can rule out a true population mean of <96.4 or >100.4, at the 0.05 significance level; but you can’t rule out a true population mean between 96.4 and 100.4 at α = 0.05. A hypothesis test would fail to reject the hypothesis that μ = 100. You can’t determine whether the true mean output of the machine is equal to 100 or not.

Leaving the symbols aside, when you test a null hypothesis your sample either is surprising (and you reject the null hypothesis) or is not surprising (and you fail to reject the null). Any null hypothesis value inside the confidence interval is close enough to your sample that it would not get rejected, and any null hypothesis value outside the interval is far enough from the sample that it would get rejected.

Special Note for Binomial Data

For numeric data, the CI and HT are exactly equivalent.

But for binomial data, the CI and HT are only approximately equivalent. Why? Because with binomial data, the HT uses a standard error derived from p_o in the null hypothesis, but the CI uses a standard error derived from p̂, the sample proportion. Since the standard errors are slightly different, right around the borderline they might get different answers. But when the hypothetical p_o is a fair distance outside the CI, as it was in the drug example, the p-value will definitely be less than α.

What about One-Tailed Tests?

Good question!

A confidence interval is symmetric (for the cases you study in this course), so it’s intrinsically two-tailed. A one-tailed HT for < or > at α = 0.01 corresponds to a two-tailed HT for ≠ at α = 0.02, so the CI for a one-tailed HT at α = 0.01 is a 98% CI, not a 99% CI. The confidence level for a one-tailed α is 1−2α, not 1−α.

If the baseline value is outside the confidence interval, you can say (at the appropriate significance level) that the true value of μ or p is different from the baseline, and then go on to say whether it’s bigger or smaller, so you get your one-tailed result.

On the other hand, if the baseline value is inside the confidence interval, you can’t say whether the true μ or p is equal to the baseline or different from it, and if you can’t say whether they’re different then you can’t say which one is bigger than the other.

10E.  Testing a Non-Random Sample

Though most hypothesis tests are to find out something about a population, sometimes you just want to know whether this sample is significantly different from a population. In this case, you don’t need a random sample, but the other requirements must still be met.

Example 16: At Wossamatta University, instructors teach the statistics course independently but all sections take the same final exam. (There are several hundred students.) One semester, the mean score on the exam is 74. In one section of 30 students, the mean was 68.2 and the SD was 10.4. The students felt that they had not been adequately prepared for the exam by the instructor. Can they make their case?

Solution: In effect, they are saying that their section performance was significantly below the performance of students in the course overall. This is a testable hypothesis. But the hypothesis is not about the population that these 30 students were drawn from; we already know about that population. Instead, it is a test whether this sample, as a sample, is different from the population.

Okay, there was a real difference. This section’s mean exam score was not only below the average for the whole course, but too far below for random chance to be enough of an explanation.

But did the students prove their case? Their case was not just that their average score was lower, but that the difference was the result of poor teaching. Statistics can’t answer that question so easily. Maybe it was poor teaching; maybe these were weaker students; maybe it was environmental factors like classroom temperature or the time of day; maybe it was all of the above.

What Have You Learned?

Chapter 11 WHYL → ← Chapter 9 WHYL

Exercises for Chapter 10

Write out your solutions to these exercises, showing your work for all computations. Then check your solutions against the solutions page and get help with anything you don’t understand.

Caution! If you don’t see how to start a problem, don’t peek at the solution — you won’t learn anything that way. Ask your instructor or a tutor for a hint. Or just leave it and go on to a different problem for now. You may find when you return to that “impossible” problem that you see how to do it after all.

Problem Set 1

Solutions → 

Problem Set 2

Solutions →