Imputation was carried out on two different samples: i) to estimate values for those among the sample of 4,810 who had information of self-harm but an incomplete set of the demographic predictors described above and ii) to impute missing data for the 4,529 participants (32.8% of the initial cohort) that were sent the relevant questionnaire but did not return it to give a useable sample of 9,268 to enable us to estimate prevalence amongst the cohort members who were sent questionnaires.
