The dependent-samples t-test (also called related-samples t-test or paired t-test) is for comparing conditions that have related participants. The most common use would be to compare a "pre", baseline condition to a "post" measurement after an intervention.
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The data values must be in pairs, like X1 and X2 are from the same person. The difference scores are calculated as post - pre scores (X2 - X1). This makes the sign of the t-test match the direction of the change over time: lower scores over time have negative t scores and vice versa.
The exact p value calculations use the jStat statistics library from Github via the Content Delivery Network.
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