A theoretical framework for measuring knowledge in screening decision aid trials

ObjectiveTo describe a theoretical framework for assessing knowledge about the possible outcomes of participating in bowel cancer screening for the faecal occult blood test.MethodsThe content of the knowledge measure was based on the UK General Medical Council's screening guidelines and a theory-based approach to assessing gist knowledge (Fuzzy Trace Theory). It comprised conceptual and numeric questions to assess knowledge of the underlying construct (e.g. false positive concept) and the approximate numbers affected (e.g. likelihood of a false positive). The measure was used in a randomised controlled trial involving 530 adults with low education, to compare the impact of a bowel screening decision aid with a screening information booklet developed for the Australian Government National Bowel Cancer Screening Program.ResultsThe numeric knowledge scale was particularly responsive to the effects of the decision aid; at follow-up decision aid participants' numeric knowledge was significantly greater than the controls (P < 0.001). This contrasts with the conceptual knowledge scale which improved significantly in both groups from baseline to follow-up (P < 0.001).ConclusionOur theory-based knowledge measure was responsive to change in conceptual knowledge and to the effect on numeric knowledge of a decision aid.Practice implicationsThis theoretical framework has the potential to guide the development of knowledge measures in other screening settings.