4.1 Introduction
What is descriptive epidemiology?
Descriptive epidemiology is a general term used to refer to a broad array of epidemiologic activities whose primary purpose is to describe disease occurrence and generate hypotheses and ideas about cause. Traditionally, this subject has been taught in terms of describing disease occurrence according to the epidemiologic variables of person, place, and time.
In contrast to descriptive epidemiology, analytic epidemiology starts with specific hypotheses about cause and then designs its studies to address these specific hypotheses. Analytic epidemiologic studies address specific hypotheses, while descriptive epidemiologic studies are more exploratory or “hypothesis generating.” With this said, it should be noted that there is no firm demarcation between descriptive epidemiology and analytic epidemiology: all epidemiologic studies serve to advance knowledge of disease causation and prevention, and many studies serve both descriptive and analytic purposes. From a learning perspective, however, it remains useful to note that studies tend to fall toward one end or the other of the descriptive-analytic spectrum.
Descriptive epidemiology often uses data from standing sources (i.e., routinely collected data). Three such sources are case series, surveillance systems, vital records, and nation health surveys.
Case series
Case series describe the medical history and clinical manifestations of a small number of individuals with a particular disease or syndrome. “Denominator data” is absent from case series. Therefore, case series cannot calculate incidence or prevalence. In addition, no referent or “control” series is present. Therefore causal conclusions are often beyond the reach of case series analysis.a Nevertheless, observations derived from cases series often signal an emerging problem and help clarify hypotheses for further investigation. An example follows.
In 1981, local clinicians and the Epidemic Intelligence Service Officer stationed at the Los Angeles County Department of Public Health prepared and submitted a report of five cases of Pneumocystis pneumonia in previously healthy young men (CDC, 1981, 2001). Before publication, editorial staff at the CDC sent the report to experts in parasitic and sexually transmitted diseases who noted that the case histories suggested that they were dealing with cellular-immune dysfunction disease acquired through sexual contact. At about the same time, the sole distributor of the antifungal drug (pentamidine) used to treat Pneumocystis pneumonia in the United States began receiving multiple requests for the medicine from physicians throughout the country. The affected individuals were, again, young men. In June 1981, CDC developed an investigative team to develop a case definition and identify risk factors for this new syndrome. Within a couple of years, a case definition for acquired immunodeficiency syndrome (AIDS) had been established and major risks factors for the condition had been identified.
Surveillance systems
Epidemiologic surveillance systems are structures set up to routinely collect and analyze data for specific types of health outcomes. Epidemiologic surveillance systems may be either active or passive in nature. Active surveillance systems require actively seeking-out cases in defined populations, and thus requires the use of specially trained personnel to retrieve and review health care and laboratory records to discover and confirm cases. In contrast, passive surveillance relies on health professionals and the public to identify cases and submit reports to the surveillance system.
Many different types of surveillance systems exist. Three examples are the Surveillance, Epidemiology and End Results program for monitoring cancer occurrence and treatment, the National Notifiable Diseases Surveillance System for monitoring reportable diseases, and the Food and Drug Administration’s MedWatch system for monitoring food and drug safety.
The Surveillance, Epidemiology and End Results (SEER) program of the National Cancer Institute is an active surveillance system that functions as the primary source of cancer statistics in the United States. SEER registries routinely collect data on patient demographics, primary tumor site, tumor morphology, stage at diagnosis, first course of treatment, and patient survival. Data from the Census Bureau are used as denominator information to calculate cancer rates within the capturement area of each of the SEER registries. SEER then compiles cancer statistics from each region to estimate cancer incidence for the entire country.
Figure 4.1 shows sharp rises in uterine cancer incidence in five regions of the United States between 1969 and 1973 (Weiss et al., 1979). Data within regions demonstrate increases of more than 10% per year over the period of observation. When the investigators further scrutinized these data, they found that the sharpest increases were among middle-aged women (data not shown). The investigators also noted that these increases paralleled large scale increases in the prescribing of estrogen for symptoms of menopause and osteoporosis that occurred concurrently, leading to a hypothesis that unopposed estrogen may increase the risk of endometrial cancer in middle-aged women. Analytic epidemiologic studies that followed this lead confirmed the association. In addition, studies in laboratory animals showed that estrogen stimulated cell proliferation of the inner lining of the uterus. Thus, the initial hypothesis raised by descriptions of increased rates were corroborated, leading to discontinuing the use of unopposed estrogen (estrogen without progestin) in post-menopausal women with intact uteri.
Figure 4.1 Figure for Illustrative Example 4.2 Endometrial cancer rates in five regions of the USA, 1969–1973. (Based on data from Weiss et al., 1976).
Aside: Some individuals may have the mistaken impression that the rates in Figure 4.1 are longitudinal. However, rates derived from open populations do not follow individual experience over time. Therefore, these data represent a series of “current” or “cross-sectional” rates—see Chapter 3 and Chapter 18 for additional information about open population current incidence rates.
The National Notifiable Disease Surveillance System provides another example of a surveillance system. This system collects reports of selected infectious and noninfectious notifiable diseases in the United States. The list of notifiable diseases is updated regularly by state legislation in collaboration with the U.S. Centers for Disease Control and Prevention.b Statistical summaries of reportable diseases are published in each volume of the Morbidity and Mortality Weekly Report (MMWR).
As a third example of a surveillance system, let us consider the U.S. Food and Drugs Administration’s MedWatch program. Since the Food and Drugs Act of 1906, the U.S. Food and Drug Administration has been the federal agency responsible for protecting the public health through the regulation and supervision of foods, cosmetics, drugs, and medical devices. The FDA instituted the Medwatch program in 1993 as a unified system by which consumers and health care professionals can voluntarily report suspected serious adverse events and product quality problems associated with the use of FDA-regulated products. Thus, Medwatch is a passive surveillance system. Because MedWatch relies on voluntary reports, its listing of cases is often incomplete. In addition, some cases may be “false positives.” Thus, like most passive surveillance systems, MedWatch is insensitive to subtle changes. Nonetheless, when used judiciously, even passive surveillance system such as the FDA’s MedWatch system can be useful in signaling problems of emerging public health threats as this example illustrates.
In this example, data from the FDA’s forerunner of the MedWatch system identified a syndrome of flank-pain and transient renal failure caused by an analgesic medication called suprofen. Figure 4.2 plots the number of flank-pain syndrome cases reported to the FDA by month of onset (open bars) and month of report (solid bars), along with marketing data for the drug (dashed-line). The passive surveillance system was stimulated by “Dear Doctor” letters, indicated by the arrows on the graph, which alerted all US physicians of the emerging problem. Identification of this unanticipated adverse reaction ultimately led to withdrawal of the drug from the market by the drug manufacturer in 1987 (Rossi et al., 1988).
Figure 4.2 Figure for Illustrative Example 4.3 Reports of flank pain and marketing data for the analgesic suprofen (Data from Rossi et al., 1988).
National health surveys and vital record systems
Governments, as part of their responsibility to monitor the health of their populations, routinely collect data on births, deaths, and various health parameters. Birth certificates are used to calculate birth rates and rates of conditions that affect the perinatal period, such as congenital malformations, birth weight, length of gestation, fetal death, and demographic characteristics of the parents. Death certificates are completed by funeral directors and attending physicians to include demographic information about the decedent and information about their cause of death. Deaths that are accident-, suicide-, or homicide-related are completed by the medical examiner or coroner as part of the investigation of the cause of death. (Local laws dictate which deaths a coroner must investigate.) In the USA, state and local registrars check the information on birth and death certificates for completeness and accuracy before forwarding copies to the National Center for Health Statistics (NCHS) for recording and compilation. Birth and mortality statistics are compiled and published in various publications, such as Vital Statistics of the United States and Health, United States. (See http://www.cdc.gov/nchs/ for a list of publications.)
In addition to tracking births and deaths, nations routinely maintain health surveys to track levels of diseases and disease determinants in populations. These surveys also include information about bodily characteristics, behavior, nutrition, health care, and other health concerns of the citizens. In the USA, the agency primarily responsible for compiling these data is the National Center for Health Statistics (www.cdc.gov/nchs). In Canada, the comparable agency is Statistics Canada (www.statcan.gc.ca). In Great Britain, the Office of National Statistics (www.statistics.gov.uk) compiles health statistics. Each of these national agencies maintains multiple health databases. Examples of survey data from the U.S. National Center for Health Statistics are the National Health and Nutrition Examination Survey (NHANES), National Health Interview Survey (NHIS), the National Hospital Discharge Survey (NHDS), National Ambulatory Medical Care Survey (NAMCS), and the National Hospital Ambulatory Medical Care Survey (NHAMCS). The methods employed by these data systems evolve over time and are documented on www.cdc.gov/nchs/.
4.2 Epidemiologic variables
Once descriptive epidemiologic data are procured, disease occurrence is tallied according to available person, place, and time variables. Person variables address characteristics and attributes of population and population subgroups. Place variables are characteristics of the locale in which people live, work, and visit. Time variables address disease occurrence in relation to various time parameters such as time since exposure, calendar time, and seasonality. Let us start by considering “person variables.”
Person
Variations in disease rates by person variables provide insights into exposures to agents and differences in host susceptibility. Table 4.1 lists examples of person variables. Two of the more common person variables are age and sex (gender), as addressed by this illustration.
Table 4.1 Examples of person variables.
| Age Sex Ethnicity/race Genetic predispositions Physiologic states (e.g., pregnancy) Concurrent disease Immune status Physical activity Marital statusDietary practices Tobacco use | Alcohol use Body mass index Host responses to social and environmental stressors Educational level Socioeconomic status Occupation Customs Religion Foreign birth Knowledge, attitudes and beliefs |
Figure 4.3 displays the age and sex distribution of nonfatal sports- and recreation-related injuries treated in emergency departments for the period July 2000 to June 2001. Rates are highest in males between the ages of 10 and 24, suggesting that special efforts to reduce injuries should be directed toward young males.
Figure 4.3 Rates of nonfatal unintentional sports- and recreation-related injuries treated in emergency departments by age and sex, USA, July 2000–June 2001.
(Source: CDC, 2002).
Two other common person variables are race and ethnicity. These factors are often related to genetic tendencies, the living habits of individuals, and the level and intensity of various social, biological, and physical environmental exposures.
Although African–Americans comprise 12% of the US population, they accounted for 33% of the tuberculosis cases reported in 1997. Twenty-three percent (23%) of the tuberculosis cases were in Hispanics and 19% were Asians and Pacific Islanders, even though these groups comprised 11 and 3.5% of the population, respectively (CDC, 2000). High rates of tuberculosis in these three groups can be explained in terms of known risk factors such as birth in a country where tuberculosis is common, HIV infection, and exposure to high-risk settings such as nursing homes, correctional facilities, and homeless shelters.
A person’s occupation is an important health determinant. People spend much of their life at work where they are exposed to chemical, physical, biological, and social stressors. Occupation is also highly correlated with socioeconomic status and specific constitutional tendencies, all of which have an influence on health.
One of the founding members of the London Epidemiological Society, William Augustus Guy (1810–1885), made this insightful observation about the rarity of cholera among brewery workers (Snow, 1855, p. 124):
… the brewers’ men seem to have suffered very lightly both in that and the more recent [cholera] epidemics. The reason of this probably is, that they never drink water, and are therefore exempted from imbibing the cholera poison in that vehicle.
Work in the brewing industry, in this instance, proved to be salubrious.
Place
Place variables are characteristics of the locale in which people live, work, and visit. Place variables may be defined in terms of geographic boundaries (e.g., street, city, state, region, country) or environmental characteristics (e.g., rural/urban, domestic/foreign, institutional/noninstitutional). Table 4.2 lists examples of host and environmental characteristics associated with place.
Table 4.2 Host and environmental factors associated with place.
| Presence and level of agents Presence of vectors that facilitate transmission Socioeconomic differences Genetic characteristics of residents Physiologic and anatomic attributes of residents Geology Climate Population density | Nutritional practices Occupations Recreational practices Urban/rural differences Economic development Social disruptions (e.g., war, natural disasters, economic downturns) Social norms in behavior Medical practices Access to health care |
Differences in the incidence and prevalence of disease by place are related to differences in host susceptibility or prevalence of causal agents.
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