Suppose that you’re comparing two equal-sized groups, Drug A and Drug B. You’ve calculated that

you need two groups of 32, for a total of 64 participants. Now, you decide to randomize group

assignment using a 2:1 ratio for A:B. To keep the same power, you’ll need

, or 48 for Drug A,

an increase of 50 percent. For B, you’ll want

, or 24, a decrease of 25 percent, for an

overall new total 72 participants in the study.

Allowing for Attrition

Sample size estimates apply to the number of participants who give you complete, analyzable data. In

reality, you have to increase this estimate to account for those who will drop out of the study, or

provide incomplete data for other reasons (called attrition). Here’s how to scale up your sample size

estimate to develop an enrollment target that compensates for attrition, remembering longer duration

studies may have higher attrition:

Enrollment = Number Providing Complete Data × 100/(100 – %Attrition)

Here are the enrollment scale-ups for several attrition rates:

Expected Attrition Increase the Enrollment by

10%

11%

20%

25%

25%

33%

33%

50%

50%

100%

If your sample size estimate says you need a total of 60 participants with complete data, and you

expect a 25 percent attrition rate, you need to enroll

, or 80 participants. That way, you’ll

have complete data on 60 participants after a quarter of the original 80 are removed from analysis.