A new drug is proposed to lower total cholesterol and a study is

designed to evaluate the efficacy of the drug in lowering cholesterol.

Fifteen patients agree to participate in the study and ea

ch is asked to take

the new drug for 6 weeks. However, before starting the treatment, each

patient's total cholesterol level is measured. The initial measurement is a

pre

-

treatment or baseline value. After taking the drug for 6 weeks, each

patient's total

cholesterol level is measured again and the data are shown

below. The rightmost column contains difference scores for each patient,

computed by subtracting the

6

-

week

cholesterol level from the baseline

level. The differences represent the reduction in tot

al cholesterol over 4

weeks. (The differences could have been computed by subtracting the

baseline total cholesterol level from the level measured at 6 weeks. The

way in which the differences are computed does not affect the outcome

of the analysis only the interpretation.)

(Chart shown below)

Because the differences are computed by subtracting the cholesterols

measured at 6 weeks from the baseline values, positive differences

indicate reductions and negative differences indicate increases (e.g.,

participant 12 increases by 2 units over 6 weeks). The goal here is to test

whether there is a statistically significant reduction in cholesterol.

Because of the way in which we compute

d the differences, we want to

look for an increase in the mean difference (i.e., a positive reduction). In

order to conduct the test, we need to summarize the differences.