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Study Design

Epidemiologic studies have traditionally been categorized as having ‘descriptive’ or ‘analytic’ designs. Descriptive studies are viewed primarily as hypothesis-generating studies and usually take advantage of routinely collected data to describe the distribution of a disease in a population in terms of the basic descriptors of person, place, and time. Analytic studies are further divided into ‘observational’ and ‘experimental’ study designs and are viewed as approaches suitable for testing specific hypotheses about disease etiology or the efficacy of disease prevention strategies. The main categories of observational studies are the cohort, case-control, nested case-control, case-cohort, case crossover, and crosssectional designs. The most commonly employed experimental designs used in epidemiologic research include the classic randomized clinical trial and the quasi-experimental nonrandomized study design used to evaluate the effectiveness of population-based disease prevention approaches.

Descriptive Epidemiology

Data Sources

Descriptive epidemiologic studies are designed to determine the distribution of a disease in a population with regard to person, place, and time. The numbers of individuals in the population who are diagnosed with or die from various diseases are obtained from sources such as vital records files, disease registries, and surveys. Death certificates provide information on the underlying cause of death and provide basic sociodemographic data on the decedent such as age, gender, race/ethnicity, marital status, and place of residence at the time of death. Birth certificates are used to study the incidence of various birth outcomes such as low birthweight (LBW) and its relationship to various parental factors such as maternal age. Birth defects registries also exist in some areas and combine data from birth certificates and reports from hospitals, physicians, and genetic testing laboratories.

Cancer registries now exist in all regions of the United States and are used to enumerate the number of total and specific forms of cancer that occur in a defined population over a specified time period. Cancer registries routinely collect information from hospital records and pathology laboratories regarding various clinical factors such as the cancer's anatomic location and histological type, clinical and pathologic stage of the disease, and information on the methods used to diagnosis the cancer. The cancer records also include data on various sociodemographic characteristics such as age, gender, race/ethnicity, marital status, and place of residence at the time of diagnosis. The address listed in the vital record or disease registry report at the time of death, birth, or diagnosis can be coded to the individual's census tract of residence using computer-based matching algorithms. The census tract reports contain data on various measures of socioeconomic status such as income and education for the geographic area in which the individual resided at the time of birth, death, or disease diagnosis. These numerator data are then combined with population denominator data to create disease incidence or death rates by person, place, or time.

Person, Place, and Time

Characteristics of persons include age, gender, race/ethnicity, marital status, and various measures of socioeconomic status such as education and income. Most diseases show distinct patterns of occurrence with regard to these personal characteristics. Breast cancer steadily increases with age until about the age of menopause at which time the age curve flattens. After the menopause, breast cancer incidence again increases with advancing age, albeit at a slower rate. These and other observations have led researchers to consider the possibility that preand postmenopausal breast cancer arise from separate etiologic processes. Numerous other examples exist regarding the relationship between disease incidence and mortality and age.

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