A paired T-test is a statistical method used to compare the means of two related groups. It is commonly applied when the same subjects are measured under two different conditions, such as before and after a treatment. This test helps determine if there is a significant difference in the means of the two sets of observations. To perform a paired T-test, the differences between each pair of observations are calculated. These differences are then analyzed to see if their mean is significantly different from zero. The result helps researchers understand the effect of an intervention or change in conditions on the subjects being studied.