an investigator is interested in the aetiology of neonatal jaundice. To study this condition she selected 100 children born with neonatal jaundice and 100 children born in the same time period and in the same hospital,who did not have the diagnosis of neonatal jaundice. She then reviewed the obstetric and delivery records of the mothers to determine various pre-natal and perinatal exposures. This is an example of?