172 PART 4 Comparing Groups

explicit criteria for what constitutes morning sickness). Then you’ll tabulate the

results in a 2 × 2 cross-tab. The table will look similar to Figure 12-1, but instead

will say “supplement” and “placebo” as the label on the two rows, and “did” and

“did not” experience morning sickness as the headings on the two columns. And

you’ll test for a significant effect with a chi-square or Fisher Exact test. So, your

sample size calculation question is: How many subjects must you enroll to have at least

an 80 percent chance of getting p

0 05

.

on the test if the supplement truly can reduce the

incidence from 80 percent to 60 percent?

You have several ways to estimate the required sample size. The most general and

most accurate way is to use power/sample-size software such as G*Power, which

is described in detail in Chapter 4. Or you can use the online sample-size calcula-

tor at https://clincalc.com/stats/samplesize.aspx, which produces the

same results.

You need to enroll additional subjects to allow for possible attrition during the

study. If you expect x percent of the subjects to drop out, your enrollment should be:

Enrollment

100

Analyzable Number

/ (

)

100

x

So, if you expect 15 percent of enrolled subjects to drop out and therefore be unan-

alyzable, you need to enroll 100 × 197/(100 – 15), or about 231 participants.