Is a 10% shortfall in sample size critical?

Steve Simon


Dear Professor Mean, I’m reviewing a paper where they did a power calculation based on 60 patients per group, but in the research study, they ended up only getting 55/58 per group. Since their sample size was much less than what they originally planned for, does this mean that the study had inadequate power?

The researchers came within 10% of their projected sample size, and you’re ready to condemn the study into oblivion? I’d hate to see your reaction when something bad actually happens.

No research study is perfect, and falling below your projected sample size is indeed a problem, but the shortfall here is so small that it is hard to get too excited about it. You need to save your outrage for more serious problems like a study where half of the patients dropped out before the final evaluation, a study which used an unvalidated measure of pain, or a study where the researchers failed to include the consulting statistician as a co-author.

One way to look at it is that when the sample size is 10% smaller than planned, you suffer through confidence intervals that are approximately 5% wider than you had originally planned for. Is that such a terrible thing?

Another way to look at it is that a reduction of sample size by 10% leads to a 3-5% drop in power. So a study that you thought had 90% power actually has 86% power. A study that you thought had 80% power actually has 76% power.

Now I would start to worry if the sample size is 30% smaller than planned (getting 42 patients when the original goal was 60). Now your confidence interval is 20% wider than you had hoped it would be and your power is 13 to 15% lower. A sample size that is half the original plan (getting 30 patients when the original goal was 60) is definitely a problem because the confidence interval is 41% wider and the power drops by 27 to 30%. So a study that you hoped would have 80% power actually only has 50% power.

Sometimes you get lucky, and the actual standard deviation that you observed in the study is much smaller than you originally thought at the planning stage, and this cancels out the loss in precision and power. But just as often, the study that has the shortfall in sample size also has an overly optimistic initial estimate of sample size during the planning phase, leading to a double whammy.

These calculations apply for a two sample t-test, but would probably produce comparable results for other scenarios such as a paired t-test, an ANOVA F-test, or a Chi-square test for testing independence in a two by two table.