# Why should I use a paired t test?

To perform this test, select Stat > Basic Statistics > Paired t.

Use this analysis to:
• Determine whether the mean of the differences between two paired samples differs from 0 (or a target value)
• Calculate a range of values that is likely to include the population mean of the differences

For example, suppose managers at a fitness facility want to determine whether their weight-loss program is effective. Because the "before" and "after" samples measure the same subjects, a paired t-test is the most appropriate analysis.

The paired t-test calculates the difference within each before-and-after pair of measurements, determines the mean of these changes, and reports whether this mean of the differences is statistically significant.

A paired t-test can be more powerful than a 2-sample t-test because the latter includes additional variation occurring from the independence of the observations. A paired t-test is not subject to this variation because the paired observations are dependent. Also, a paired t-test does not require both samples to have equal variance. Therefore, if you can logically address your research question with a paired design, it may be advantageous to do so, in conjunction with a paired t-test, to get more statistical power.

The paired t-test also works well when the assumption of normality is violated, but only if the underlying distribution is symmetric, unimodal, and continuous. If the values are highly skewed, it might be appropriate to use a nonparametric procedure, such as a 1-sample sign test.

For Paired t, the hypotheses are:
Null hypothesis
 H0: μd = μ0 The population mean of the differences (μd) equals the hypothesized mean of the differences (μ0).
Alternative hypothesis
Choose one:
 H1: μd ≠ μ0 The population mean of the differences (μd) does not equal the hypothesized mean of the differences (μ0). H1: μd > μ0 The population mean of the differences (μd) is greater than the hypothesized mean of the differences (μ0). H1: μd < μ0 The population mean of the differences (μd) is less than the hypothesized mean of the differences (μ0).
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