Suboptimal adherence to medical treatments is prevalent across several clinical conditions and can lead to treatment failure. Adherence is a far from fully explored phenomenon and there is little knowledge about how patients interpret treatment effects. Commonly used treatment evaluation measures are often relative measures, which may be difficult for lay people and patients to understand.

The overall aim of this thesis was to investigate factors with relevance to adherence, to estimate treatment effects with the time-based Delay of Event (DoE) measure in anticoagulant preventive treatments, and to explore how lay people responded to the DoE measure, as compared with established measures, regarding treatment decisions and effect interpretation.

A quantitative population-based cross-sectional design was used for Study I. Study II used data from the Apixaban for Reduction in Stroke and Other Thromboembolic Events in Atrial Fibrillation (ARISTOTLE) clinical trial and estimated effects as DoEs. Studies III and IV were carried out as randomised survey experiments.

The results showed that general adherence behaviour was associated with both environmental and social factors. Estimations of DoE showed that stroke or systemic embolism was delayed 181 (95% CI 76 to 287) days through twenty-two months of apixaban use, as compared with  warfarin use. The delay of major and intracranial bleeding was 206 (95% CI 130 to 281) and 392 (95% CI 249 to 535) days, respectively, due to apixaban use for twenty-two months, as compared with  warfarin use. Presenting preventive treatment effects as DoEs to lay people was associated with high willingness to initiate treatment and positive views on treatment benefits and willingness to pay for treatment.

Non-optimal adherence was partly associated with modifiable factors and it might be possible to increase adherence by managing these factors. Estimations of DoEs in preventive treatments gave information on effects regarding delay of different outcomes; the estimation also provides tools that might be useful for interpreting and communicating treatment effects in clinical decision-making. Lay people seemed to react rationally to variations in DoE magnitude; a higher proportion accepted treatment when the magnitude was greater.