Evidence-appraisal glossary
Run-In Period
A phase before randomization in which candidates receive a placebo or the active drug, so the trial can screen out poor adherers or non-responders before the real comparison begins. It can sharpen a trial's power but narrow who the results apply to.
Also called: lead-in period.
A run-in can select a more adherent or better-tolerating group, which raises the chance of detecting a true effect and cuts later dropout. The cost is generalizability: results from a pre-filtered population may overstate how well a treatment works in everyday patients who were never screened. It is worth checking whether an impressive effect was helped along by an active run-in that quietly removed likely non-responders.
Read the full Reading the Evidence blog.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.