OVERVIEW OF STUDY DESIGN. COMMUNITY SURVEYS Nigel Paneth

THREE MAJOR EPIDEMIOLOGIC STUDY TYPES COHORT STUDIES CASE-CONTROL STUDIES CROSS-SECTIONAL STUDIES

COHORT STUDIES A. Sample selection by exposure Example: following a group of workers exposed to asbestos and comparing their lung cancer rates to a group of workers not exposed to asbestos. If the non-exposed workers are matched to the exposed workers, the design is sometimes called exposure-control. B. Sample selection not by exposure Example: following all workers, or a random sample of people in the community, determining their exposure to asbestos, then following them up for lung cancer.

CASE-CONTROL STUDIES C. Sample selection by outcome Example: workers with lung cancer are selected, and so are workers without lung cancer. Asbestos exposure is compared in the two groups. D. Sample selection not by outcome (virtually never done) Example: All workers in a plant are assessed for presence or absence of lung cancer. The workers with and without lung cancer are compared for asbestos exposure.

CROSS-SECTIONAL STUDIES E. Sample selection by exposure Example: Workers with and without exposure to asbestos are compared for the simultaneous presence of lung cancer. F. Sample selection by outcome   Example: Workers determined to have or not have lung cancer at a point in time are compared for simultaneous exposure to asbestos

G. Sample selection neither by outcome nor by exposure   Example: All workers in a plant are simultaneously assessed both for asbestos exposure and lung cancer status. (Sampling schemes D, E and G are likely to be inefficient)

THE MAIN DESCRIPTIVE STUDY IN EPIDEMIOLOGY: THE COMMUNITY SURVEY A study which attempts to ascertain the frequency of a disease in a fixed geographic region (ideally) or in a group defined by a common membership, e.g. school-children. It usually assesses the frequency of disease in easily ascertained sub-groups, i.e. age, gender, geographic sub-units, ethnicity. etc. Often it is the first step in looking at risk factors for disease.

THE COMMUNITY SURVEY PURPOSES 1. EXAMINE PREVALENCE (MORE RARELY, INCIDENCE) 2. ESTABLISH A BASELINE FOR FUTURE STUDIES 3. SET PUBLIC HEALTH PRIORITIES

4. TARGET POPULATIONS AT RISK 5. NOTICE GEOGRAPHIC CLUSTERING 6. OBTAIN BASIC EPIDEMIOLOGY: TIME, PLACE, PERSON (RACE, SEX, AND OTHER EASILY OBSERVED HOST FACTORS) 7. USUALLY NOT DRIVEN BY AN ETIOLOGIC HYPOTHESIS-DRIVEN.

II. METHODS 1. DIRECTIONALITY: COMMONLY CROSS-SECTIONAL, THOUGH CAN INCORPORATE COHORT AND CASE-CONTROL COMPONENTS 2. SAMPLING: GENERALLY POPULATION. 3. TIMING: USUALLY CONCURRENT

III. TARGET CONDITIONS 1. SPECIFIC DISEASES 2. HANDICAPPING CONDITIONS 3. SYMPTOMS 4. RISK FACTORS FOR DISEASE 5. HAZARDOUS EXPOSURES

IV. TECHNIQUES 1. ACCESSING THE POPULATION: A. HOUSE-TO-HOUSE B. TELEPHONE C. MAILED SURVEY

2. ASSESSMENT A. SELF-REPORT B. PHYSICAL EXAMINATION C. LABORATORY TESTS D. COMBINATIONS