Strengths and limitations
The intended sample size was recruited, with a high accrual rate; participants were representative of the local population. A hierarchical approach was applied to define the primary outcome using objective health services data in preference to self-report, and self-reported information was relied on for only three participants for the primary analysis. To maintain blinding, risk-appropriate screening was defined based on the risk assessment method specific to each trial group. There were complete data for the primary outcome for 99% of trial participants. The sensitivity analyses showed the findings were robust to different assumptions.Including participants who were not due CRC screening during follow-up diluted the observable effect of the intervention. They were included because in the current study the authors were interested in effects on risk-appropriate screening, both under- and overscreening, in average and increased risk groups. The rate of risk-appropriate screening in the control group who were due screening was only 39%, similar to rates of participation in the national screening programme. It was not possible to confirm whether someone was genuinely due screening in the 12-month follow-up until the authors obtained the objective screening data from health records and knew their baseline risk. The preliminary estimates of baseline risk-appropriate screening, on which the original and revised sample sizes were based, could not adequately account in advance that only 50% of the sample were due CRC screening during the follow-up period. Although guidelines recommend CRC screening in those at increased risk from age 40 years, a decision was made to recruit a sample aged 50–74 years. If a younger cohort had been recruited, an even larger proportion of participants who were not due screening during follow-up would have been included in the study.