CHAPTER 25 Ten Easy Ways to Estimate How Many Participants You Need 367

attrition). Here’s how to scale up your sample size estimate to develop an enroll-

ment 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 60

1 33

.

, or 80

participants. That way, you’ll have complete data on 60 participants after a quar-

ter of the original 80 are removed from analysis.