OBJECTIVE: To investigate whether eligibility criteria that exclude the elderly, persons with psychiatric disease, and persons with substance abuse problems from participation in randomized controlled trials (RCTs) are subjective and hence a source of variability in enrolment decisions and investigator uncertainty.

DESIGN: Survey questionnaire.

PARTICIPANTS: Cancer investigators from the United States and Canada.

INTERVENTIONS: Investigators were presented with clinical vignettes from 3 patient categories--eligible, ineligible and uncertain--for each of 5 eligibility criteria--3 subjective and 2 objective--and were asked whether they would enrol the patient in a trial and how sure they were of this decision. Demographic characteristics of the investigators were also collected.

OUTCOME MEASURES: The difference in enrolment decisions between subjective and objective criteria, and the difference in the certainty associated with these decisions.

RESULTS: Of 365 questionnaires sent out, 224 usable ones were returned. Compared with the objective criteria, the subjective criteria were associated with more variable enrolment decisions (p = 0.07 for the "eligible" scenario and p = 0.0001 for the "ineligible" and "uncertain" scenarios), and investigators were less sure about the decisions they made (p = 0.0001 for all scenarios). Demographic characteristics of the investigators failed to explain the observed differences.

CONCLUSIONS: Subjective eligibility criteria may interfere with the conduct and interpretation of RCTs and, therefore, their use ought to be justified explicitly in the study protocol. RCT designers, funding agencies and research ethics boards have an important role in reviewing eligibility criteria for their necessity.