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 twice, 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. The paired t-test calculates the differences between each pair of observations and then assesses whether the average of these differences is significantly different from zero. It assumes that the differences are normally distributed, making it suitable for small sample sizes when this condition is met.