The use of self-report may have encouraged more honest answers than an interview situation [28], although the stigma of self-harm may still have led to underreporting and therefore an underestimation of prevalence. ALSPAC has very strict procedures in terms of anonymity and confidentiality, with which study participants are familiar, which it is hoped will have minimised this possibility. A second problem with using self-report is that participants are defining self-harm for themselves. The free text responses regarding what they did gave some insight into the behaviours that participants included - of the 147 who gave such a response only 9 were excluded. However, for those who did not include a free text response, we were reliant on their own decisions as to what constituted self-harm. Relatedly, respondents who had depressed mood may have interpreted their own behaviour and underlying motivations in more negative ways, which may have introduced bias in the association seen between depressed mood and self-harm, particularly self-harm with intent to die. A third problem, relating to the face validity of the questionnaire, is that approximately 10% of
