Purpose: Randomised controlled trials traditionally investigate treatment effects but can also be used to estimate selection effects (the self-selection of one treatment over another) and preference effects (the effect of receiving the preferred treatment). This study illustrates a method (Rucker 1989 Statist. Med.) to estimate treatment, preference and selection effects to investigate whether informed choice supported by a decision aid is beneficial compared to policy directed management (limited patient choice).
Method: The method is illustrated using data from the IMAP trial, which was designed to investigate the psychosocial outcomes over 1 year of an informed choice between HPV triage or usual care by repeat Pap smear compared to policy directed management of each option. We used a 3-arm trial design with patients randomised to either one of two treatments (limited choice) or to an informed choice arm. The method is unique in that it allows the effects of treatment, preference (i.e. choice) and selection (selection bias) to be estimated separately. Information from the choice arm is used to estimate effects within the randomised arms for those who did and did not receive their preferred treatment.
Results: With traditional analysis those in the HPV arm were more satisfied than those in the Pap arm, with little difference between informed choice and HPV. There was little difference in quality of life (SF36) scores between the three arms. The Rucker analysis showed weak evidence for an effect of preference on the SF36 scores: mental health score (6.0, 95% CI-0.6 to 12.9, P= 0.07) with choice associated with improved quality of life. There was evidence of a selection effect for the satisfaction of women with their health care in general and with the care of their abnormal Pap, with women who selected or would have selected HPV being less satisfied than those who selected or would have selected Pap triage (-2.1 95% CI-4.0 to -0.3, P=0.02 and -1.2, 95%CI-2.5 to -0.2, P=0.03).
Conclusions: The Rucker method should be used to estimate the effect of informed choice compared to policy or clinician directed management (ie. limited patient choice) as it brings important additional information to the interpretation of trial data.
Candidate for the Lee B. Lusted Student Prize Competition