Recruiting, retaining and gathering complete data on participants in research projects, be they patients or health professionals, can be extremely difficult. These problems increase the risk that research will be abandoned before its true value is appreciated, or lead to delays in resolving uncertainty for decision makers, while further studies are done. Poor recruitment, retention and outcome collection frequently lead to many prospective studies being extended, increasing costs. Researchers need to use strategies that are themselves evidence-based. This study proposes to link with an existing longitudinal ageing study called NICOLA to provide evidence on what research participants prefer in relation to providing personal information through a self-completed questionnaire. NICOLA is a large study of people over the age of 50 that is being conducted in Northern Ireland. NICOLA is aiming to recruit 8500 people and will ask them questions about participation in social activities, including organised structured and informal activities; relationship quality; loneliness; stress; resilience; quality of life; alcohol intake; food poverty and assess their health and wellbeing. Participants in NICOLA agree to having an interviewer visit them at home to ask questions about their lives, complete questionnaires in their own time and attend a health assessment appointment. They also agree to being followed up over a course of at least 10 years. The research described here will examine the impact of differing times and formats of a self assessment questionnaire on completion rates, specifically: