In the Evidence worksheet, we did the following t-test:

t.test(cpsdata$income ~ cpsdata$sex)

    Welch Two Sample t-test

data:  cpsdata$income by cpsdata$sex
t = -3.6654, df = 9991.3, p-value = 0.0002482
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
 -14518.10  -4400.64
sample estimates:
mean in group female   mean in group male 
            82677.29             92136.66

Here’s a more detailed explanation of the output of that test – we’ll go through each bit:

What you did

Welch - There’s more than one way to do a t-test. R uses the method recommended by Welch. The Welch method is always a better choice than doing the standard (a.k.a. “Student”) t-test.

Two Sample t-test - It’s two sample because you have two different groups (“samples”) of people being compared in the test – females and males.

data: cpsdata$income by cpsdata$sex - This just reminds you what data you’re analyzing, it’s basically a copy of what you told it to do, i.e. cpsdata$income ~ cpsdata$sex

alternative hypothesis: true difference in means is not equal to 0 - This is a way of saying that, before looking at the data, you made no assumptions about whether men would earn more than women, or vice versa. This is sometimes called a two-tailed test - see below if you can safely assume a direction before looking at your data (also called a one-tailed test).

What you found

t = -3.6654 - This is the t value – the output of a t-test. It’s a bit like an effect size, except it’s harder to interpret, because its value is also affected by the sample size (larger samples mean larger t values, other things being equal). A t value isn’t at all useful on its own but along with the degrees of freedom (see below), we can use it to calculate the p value (also see below). The t-value is negative for the reason explained in the one-tailed tests section, below. Generally speaking, psychologists ignore the minus sign when reporting t values in their papers, although people differ on this.

df = 9991.3 - df is short for degrees of freedom. In a “Student” t-test, the degrees of freedom is the sample size, minus the number of means you’ve calculated from that sample. In a Welch t-test, this number is corrected to deal with some of the problems with the Student’s t-test. This correction makes the Welch t-test more accurate than the Student’s t-test.

p-value = 0.0002482 - The is the p value of the t-test. It’s the probability of your data, under the assumption there is no difference between groups (sometimes called the null hypothesis). You need the t value and the degrees of freedom to be able to calculate the p value … but R does those calculations for you.

Group means

Although you can get these in other ways, for convenience the t.test command gives you the mean for each group:

sample estimates:
mean in group female   mean in group male 
            82677.29             92136.66

These are the mean incomes for the two groups, $82677.29 for females and $92136.66 for males. In our sample, women earn about $9459 less than men, on average.

How big is this difference likely to be in the US population as a whole — assuming our sample is representative of the US population? This is where this part of the ouput comes in:

Confidence intervals

95 percent confidence interval:
 -14518.10  -4400.64

This 95% confidence interval tells us that the mean difference in the population is very likely to be somewhere between $14,518.10 and $4400.64. If we had collected more data we could have been more precise.

The 95% confidence interval is the only thing reported by a t-test that is both useful and easy to interpret. Psychologists are now encouraged to report it in their papers, like this:

Women earned less than men, [-$14518, -$4400], d = .20, t(991.3) = 3.67, p < .05.

One-tailed tests

In a one-tailed test, you decide before looking at your data which direction the effect should be in. For example, you may have read a lot of scientific papers about the gender pay gap, so you’re pretty sure that if you find a difference in your sample, it’ll be the women who earn less.

R deals with groups in alphabetical order of the label you gave them, so females are group 1, and males are group 2 (because f comes before m in the alphabet). You expect the group 1 mean to be less than the group 2 mean. So you use this command to do this one-tailed test:

t.test(cpsdata$income ~ cpsdata$sex, alternative = "less")

The t value is negative because R calculates the mean difference as group 1 minus group 2.

If instead your hypothesis was that females earn more than males, you’d use this command instead:

t.test(cpsdata$income ~ cpsdata$sex, alternative = "greater")

Licence

This material is distributed under a Creative Commons licence. CC-BY-SA 4.0.