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The Dependent-Samples Test

27 November, 2015 - 16:54

The dependent-sampletes(sometimes called the paired-samples test) is used to compare two means for the same sample tested at two different times or under two different conditions. This makes it appropriate for pretest-posttest designs or within-subjects experiments. The null hypothesis is that the means at the two times or under the two conditions are the same in the population. The alternative hypothesis is that they are not the same. This test can also be one-tailed if the researcher has good reason to expect the difference goes in a particular direction.

It helps to think of the dependent-samples test as a special case of the one-sample ttest. However, the first step in the dependent-samples ttest is to reduce the two scores for each participant to a single differencscorby taking the difference between them. At this point, the dependent- samples ttest becomes a one-sample test on the difference scores. The hypothetical population mean (\mu_0) of interest is 0 because this is what the mean difference score would be if there were no difference on average between the two times or two conditions. We can now think of the null hypothesis as being that the mean difference score in the population is 0 (\mu_0 = 0) and the alternative hypothesis as being that the mean difference score in the population is not 0 (\mu_0 ≠ 0).