To evaluate the effectiveness of a treatment, researchers will often measure individuals before and after treatment, and record the amount of difference between the two scores. If the differences average around zero, it is an indication that the treatment has no effect. However, if the differences are consistently positive (or consistently negative), it suggests that the treatment does have an effect. For example, Kerr, Walsh, and Baxter (2003) evaluated the effectiveness of acupuncture treatment by measuring pain for a group of participants before and after a 6-week treatment program. Hypothetical data, similar to the results of the study, are as follows. Note that each score measures the difference in pain level after treatment compared to the pain level before treatment.
-3, -4, -3, -2, -5, -4, -6, +1, -4, -3, -4, -5, -4, -2, -5, -3, +2, -4, -1, 0
This tutorial provides guidelines on creating a histogram showing the frequency distribution for different scores.