Past exam of the mathematics course of the University of Cambridge / 2007 / iii / Paper 46 / 2 / d / Solution

Measure receipt and transfers directly rather than inferring them only from the outcome difference. Use identifiable pack codes and confidential early follow-up to record whether packs were retained, lost, taken, given away or administered, and to whom. Where feasible, corroborate administration reports with care records and establish whether recipients belonged to the opposite randomized group.

A process-evaluation sample across both arms can estimate the frequency and direction of treatment contamination. Separate receipt tracking from blinded outcome ascertainment where possible. Keep randomized assignments for the primary intention-to-treat analysis: excluding transfers or reclassifying participants by receipt can introduce confounding, since receipt behavior is not randomized. Receipt data can inform a supplementary analysis whose additional assumptions are explicit.