Format and readability of an enhanced invitation letter did not affect participation rates in a cancer registry-based study: a randomized controlled trial

Objectives: To assess the effectiveness of an “enhanced” invitation letter in increasing participation in an Australian cancer registry-based study and assess the representativeness of the study sample. Study Design and Setting: Eight hundred hematological cancer survivors, diagnosed within the last 3 years and aged 18–80 years at recruitment, were selected from one Australian state-based cancer registry. Half were randomly allocated to receive the standard invitation letter (control group). The remaining half received a modified invitation letter, incorporating content and design characteristics recommended to improve written communication (intervention group). Results: Of the 732 eligible survivors, 268 (37%) returned a completed survey. There was no difference in participation between the intervention (n = 131, 36%) and control groups (n = 137, 38%; P = 0.53). Participants were representative of the population for characteristics assessed, except for age group at diagnosis. Survivors 50 years or older at diagnosis had higher odds of returning a completed survey, 50–59 (odds ratio [OR]: 2.53; 95% confidence interval [CI]: 1.47, 4.35), 60–69 (OR: 2.69; 95% CI: 1.58–4.58), and 70–80 (OR: 1.90; 95% CI: 1.07–3.35), than survivors aged 15–39 years at diagnosis. Conclusion: An enhanced invitation letter was not effective in increasing participation of hematological cancer survivors in an Australian cancer registry study. The study sample was moderately representative on variables assessed, with age group at diagnosis the only variable associated with participation. Research should evaluate strategies to increase participation in registry studies and focus on tailoring techniques to patient's age.