We’ve had some questions about how to answer a couple of questions related to a study’s planned number of subjects, and how to address enrollment in the consent form. See the questions below:
The first question, “What is your enrollment goal?” is asking “How many subjects do you need to have in your study to be able to meet your scientific goals?”
The second question asks, “How many people will you need to screen to be able to reach the above enrollment goal?” So, in a study where you will be obtaining subject consent, the second number will always be at least as large as the first, and usually a bit bigger, to account for screen failures.
How should these two numbers be addressed in consent forms? You could say, “We expect to screen 50 people to find 30 who qualify for our study.”
Remember, for the IRB’s purposes, once a subject has signed a consent form, that person should be counted as “enrolled” in continuing review reports, even if that person fails screening and does not progress into the study. We know some funding agencies count enrollment differently — they don’t consider a subject “enrolled” until that person has passed screening and has started in the main study. So you may, in a sense, be keeping two sets of books to track enrollment in a single study.