Interval and clinical cohort studies: epidemiological issues.

Cohort studies based upon clinic populations and medical records are becoming more abundant due in part to an increasing trend toward electronic medical records and advancement in information technology. This design has been utilized in the HIV setting to great success and involves following individuals as they access medical care. These clinical cohort designs have not been compared to the classic interval cohort design in which individuals are followed at specified intervals that are unrelated to the participants' ongoing health care. The interval and clinical cohort designs are distinguished and the advantages and disadvantages inherent in each design are discussed.