Timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised DIagnostic Assessment for children and adolescents with emotional difficulties) Trial.

To investigate the effects of the timing and value of participant vouchers on questionnaire return rates in a randomised trial involving children and young people and their parent/carers. Participants will be randomised to receive a single £20 voucher, conditional on completion of the 12-month questionnaires or up to two £10 vouchers, conditional on questionnaire completion at each of the 6- and 12-month time-points.