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Table 1 Principal strata and sources of (non)compliance

From: Intermediate outcomes in randomized clinical trials: an introduction

  Experimental group (calcium) Control group (placebo)
Source of (non) compliancea Taking >80% of tablets assigned Physician indicated collateral treatment involving substantial calcium Subject self-indication collateral treatment involving substantial calcium Taking >80% of tablets assigned Physician indicated collateral treatment involving substantial calcium Subject self-indication collateral treatment involving substantial calcium
Strata       
Compliers Yes No No Yes No No
  Yes Yes No Yes Yes No
  Yes Yes Yes Yes Yes Yes
Always- takers Yes No Yes Yes No Yes
  No Yes No No Yes No
  No Yes Yes No Yes Yes
  No No Yes No No Yes
Never-takers No No No No No No
Defiers*       
  1. *As the trial is blinded, there are no defiers, as subjects cannot systematically take the opposite treatment. If a subject decides not to take the pills (treatment) assigned, she belongs to the never-takers stratum. Compliers are subjects who always take the treatment assigned.
  2. aSources of (non)compliance: 1, taking >80% of pills assigned; 2, collateral treatment involving substantial calcium indicated by physician; 3, self-indicated collateral treatment involving substantial calcium.