Research in which patients are studied from their first contact with a doctor or hospital regarding their illness – as opposed to a retrospective study, in which case notes are analysed for a group of patients already treated or under treatment. The advantage is that before the study is undertaken, decisions can be made on what criteria determine who should enter the study, what information to collect and how many patients are needed to provide a meaningful result. The results of prospective studies are regarded as likely to be far more accurate than those from retrospective studies, where missing data and both researcher and patient BIAS can colour the results.