The opposite of a PROSPECTIVE STUDY, involving review of the case notes and/or interviewing or sending a questionnaire to a collection of people (often those who have been diagnosed with the same condition) to assess the effects of the illness, that is its MORBIDITY. The procedure is commonly used in studying the EPIDEMIOLOGY of disease. It is regarded as likely to provide less reliable results than a prospective study, in which patients are recruited when they first come to the researcher's or doctor's attention.