A run-in phase or run-in period describes the period before the trial starts when all possible study participants are given the placebo, or the medication, or sometimes both, to screen out certain patients. Data from run-in periods are used for two reasons: to screen out participants who are not eligible based on conditions or exclusions or who show that they are unwilling or unable to be compliant with the intervention, or as a “washout” period to allow the effects of a previous intervention to fade before starting a new one.
Deeper dive
Sometimes run-in phases sharpen a study’s findings in important ways, but they can also skew the results. For example, patients might be asked to take a placebo to screen out patients who are irregular pill takers. This might result in a study of highly motivated patients, who aren’t necessarily representative of the real world. Other run-in phases are meant to screen out patients who report side effects early on, perhaps decreasing the true population of people who experience side effects on the drug. Ask about the purpose of the run-in phase and find out whether the results from the run-in were included in the final analysis.