OBJECTIVES: To determine whether randomised controlled trials (RCTs) lead to the same effect size and variance as non-randomised studies (NRSs) of similar policy interventions, and whether these findings can be explained by other factors associated with the interventions or their evaluation. DATA SOURCES: Two RCTs were resampled to compare randomised and non-randomised arms. Comparable field trials were identified from a series of health promotion systematic reviews and a systematic review of transition for youths with disabilities. Previous methodological studies were sought from 14 electronic bibliographic databases (Applied Social Sciences Index and Abstracts, Australian Education Index, British Education Index, CareData, Dissertation Abstracts, EconLIT, Educational Resources Information Centre, International Bibliography of the Sociological Sciences, ISI Proceedings: Social Sciences and Humanities, PAIS International, PsycINFO, SIGLE, Social Science Citation Index, Sociological Abstracts) in June and July 2004. These were supplemented by citation searching for key authors, contacting review authors and searching key internet sites. REVIEW METHODS: Analyses of previous resampling studies, replication studies, comparable field studies and meta-epidemiology investigated the relationship between randomisation and effect size of policy interventions. New resampling studies and new analyses of comparable field studies and meta-epidemiology were strengthened by testing pre-specified associations supported by carefully argued hypotheses. RESULTS: Resampling studies offer no evidence that the absence of randomisation directly influences the effect size of policy interventions in a systematic way. Prior methodological reviews and meta-analyses of existing reviews comparing effects from RCTs and non-randomised controlled trials (nRCTs) suggested that effect sizes from RCTs and nRCTs may indeed differ in some circumstances and that these differences may well be associated with factors confounded with design. No consistent explanations were found for randomisation being associated with changes in effect sizes of policy interventions in field trials. CONCLUSIONS: From the resampling studies we have no evidence that the absence of randomisation directly influences the effect size of policy interventions in a systematic way. At the level of individual studies, non-randomised trials may lead to different effect sizes, but this is unpredictable. Many of the examples reviewed and the new analyses in the current study reveal that randomisation is indeed associated with changes in effect sizes of policy interventions in field trials. Despite extensive analysis, we have identified no consistent explanations for these differences. Researchers mounting new evaluations need to avoid, wherever possible, allocation bias. New policy evaluations should adopt randomised designs wherever possible.