Investigators telephone and visit hospitals and clinics to collect information on any further cases. What is this method of case finding called?

1. Case finding usually starts with the places where a diagnosis is likely to be made: clinics, hospitals and laboratories. 2. Sending a letter that describes the situation and asks for reports is stimulated or enhanced passive surveillance, because the facility still decides to report. 3. Telephoning or visiting to collect the information directly is active surveillance, because the investigator goes and gets it. 4. The distinction is who does the reaching out, so passive is the wrong label here. 5. A cross-sectional survey measures exposure and outcome at one moment in a sample, which is not what is described. 6. A retrospective cohort compares attack rates in exposed and unexposed groups, which comes later, at hypothesis testing. _Source: CDC Principles of Epidemiology in Public Health Practice, 3rd Edition, section Step 5 Find Cases Systematically and Record Information_