Chapter Contents
The Descriptive Triad—Or Pentad? 14
Five ‘W’ Questions 14
Types of Descriptive Studies 15
Case Report 16
Case-Series Report 16
Cross-Sectional (Prevalence) Studies 16
Surveillance 17
Ecological Correlational Studies 18
Uses of Descriptive Studies 18
Advantages and Disadvantages 19
Overstepping the Data 20
Conclusion 20
Descriptive studies often represent the first scientific toe in the water in new areas of inquiry. A fundamental element of descriptive reporting is a clear, specific, and measurable definition of the disease or condition in question. Like newspapers, good descriptive reporting answers the five basic ‘W’ questions: who, what, why, when, where … and a sixth: so what? Case reports, case-series reports, cross-sectional studies, and surveillance studies deal with individuals, whereas ecological correlational studies examine populations. The case report is the least publishable unit in medical literature. Case-series reports aggregate individual cases in one publication. Clustering of unusual cases in a short period often heralds a new epidemic, as happened with AIDS. Cross-sectional (prevalence) studies describe the health of populations. Surveillance can be thought of as watchfulness over a community; feedback to those who need to know is an integral component of surveillance. Ecological correlational studies look for associations between exposures and outcomes in populations (e.g., per capita cigarette sales and rates of coronary artery disease) rather than in individuals. Three important uses of descriptive studies include trend analysis, healthcare planning, and hypothesis generation. A frequent error in reports of descriptive studies is overstepping the data; studies without a comparison group allow no inferences to be drawn about associations, causal or otherwise. Hypotheses about causation from descriptive studies are often subsequently tested in rigorous analytical studies.
Descriptive studies play several important roles in medical research. They are often the first foray into a new disease or area of inquiry—the first scientific ‘toe in the water’. They document the health of populations and often prompt more rigorous studies. Because descriptive studies are common, clinicians need to know their uses, strengths, and weaknesses.
A descriptive study is ‘concerned with and designed only to describe the existing distribution of variables without much regard to causal relationships or other hypotheses’. The key qualifier about causal hypotheses is sometimes forgotten by investigators, resulting in erroneous conclusions. Here, we provide an overview of the advantages and disadvantages of descriptive studies, provide examples of several types of descriptive study, examine their clinical uses, and show how they can be misinterpreted.
The Descriptive Triad—Or Pentad?
Five ‘W’ questions
Traditional descriptive epidemiology has focused on three key features: person, place, and time, or, in the infectious disease model, agent, host, and environment. An alternative way of thinking about descriptive epidemiology is newspaper coverage. Good descriptive research, like good newspaper reporting, should answer five basic ‘W’ questions—who, what, why, when, and where—and an implicit sixth question: so what?
Who Has the Disease in Question?
Age and sex are universally described, but other characteristics might be important too, including race, occupation, or recreational activities. The risk of venous thromboembolism, for example, increases progressively with age. Breast cancer is rare in men, who account for only 1% of cases. However, Klinefelter syndrome (47,XXY chromosome complement) increases the risk more than 20-fold. Black women have two to three times the risk of leiomyomas of the uterus as do white women. Percival Pott’s discovery of the association between chimney sweeps and scrotal cancer related to soot is a classic in occupational epidemiology. The phrase ‘mad as a hatter’ stems from mercury-induced psychosis in workers with occupational exposure to this heavy metal in the hat industry. Commercial fishing remains a dangerous occupation, and driving all-terrain vehicles or snowmobiles while drunk is a dangerous way to have fun.
What Is the Condition or Disease Being Studied?
Development of a clear, specific, and measurable case definition is an essential step in descriptive epidemiology. Without such a description, the reader cannot interpret the report. Some conditions, such as fractures, can be overt. Other diagnoses might be challenging: multiple sclerosis, systemic lupus erythematosus, and pelvic inflammatory disease (salpingitis), for example. By use of the consensus or Delphi panel approach rather than empirical evidence, some organisations have promulgated case definitions that have subsequently been shown to be invalid.
Stringent criteria for case definitions are desirable. Admittedly, if only the more severe cases of disease are targeted, milder or earlier cases will be missed. Although this approach inevitably leads to some underreporting, the trade-off is better specificity; severe cases of a disease are less likely to be confused with other conditions than are mild cases. An example would be the stringent case definition used for toxic shock syndrome, which requires involvement of multiple organ systems. In recent decades, the Centers for Disease Control and Prevention has repeatedly revised its case definition of HIV infection, which in turn influences the reported incidence and prevalence of the disease.
Why Did the Condition or Disease Arise?
Descriptive studies often provide clues about cause that can be pursued with more sophisticated research designs ( Panel 2.1 ).
| Clinical Observation | Underlying Association |
|---|---|
| Hepatocellular adenoma in young women | Exposure to high-dose oral contraceptives |
| Blindness in newborn infants | High ambient oxygen concentrations in incubators |
| Kaposi sarcoma in young men | Infection with HIV-1 |
| Angiosarcoma of the liver in employees | Industrial exposure to vinyl chloride |
| Cataracts, heart defects, and deafness in newborns | Maternal infection with rubella during pregnancy |
| Gout | Lead nephropathy among plumbers and painters |
When Does the Condition Occur?
Time provides important clues about health events. The prototype might be the outbreak of food poisoning within hours after ingestion of staphylococcal toxin. Some temporal relations can be long, such as mesothelioma decades after asbestos exposure. Furthermore, cervical and other epithelial cancers develop decades after infection with human papillomavirus, and births and infections such as pneumonia and influenza have regular seasonal patterns. Iatrogenic morbidity in teaching hospitals has seasonal variation as well, when new trainees arrive in July. (Readers should try to schedule illnesses later in the academic year.)
Where Does or Does Not the Disease or Condition Arise?
Like age, geography influences health. Proximity to rodents and insects (and their parasites) has shaped both medical and political history (e.g., the Black Plague). Despite high levels of public hygiene in ancient Rome, including public baths and toilets ( Fig. 2.1 ), gastrointestinal parasites were common in the population. Living downwind from a smelter or drinking city water in Flint, Michigan, can lead to lead poisoning. The prevalence of malaria is inversely related to elevation above sea level, and Zika virus infection has emerged as another mosquito-borne illness of importance; weather patterns may influence its spread.
So What?
The implicit ‘W’ relates to the public health effect. Is the condition a current and timely one? Is it serious? Are large numbers involved? Are its societal implications broad? Has it been studied before? Although many descriptive reports herald new illnesses or monitor health, the net effect of others might be only thicker curricula vitae at the expense of thinner forests.
Types of Descriptive Studies
Descriptive studies consist of two major groups: those that deal with individuals and those that relate to populations. Studies that involve individuals are the case report, the case-series report, cross-sectional studies, and surveillance, whereas ecological correlational studies examine populations.
Case report
The case report is the least publishable unit in the medical literature. Often, an observant clinician reports an unusual disease or association, which prompts further investigations with more rigorous study designs (see Panel 2.1 ). For example, clinicians reported benign hepatocellular adenomas, a rare tumour, in women who had taken oral contraceptives. A large case-control study pursued this lead and confirmed a strong association between long-term use of high-dose pills and this rare, but sometimes deadly, tumour. Not all case reports deal with serious health threats, however; some simply enliven the generally bland medical literature. Examples include joggers’ nipples, chicken bones in the uterus, and scrotal burn from a laptop computer.
Some journals refuse to publish case reports, perhaps because they can drag down the journal’s impact factor. In response, the number of journals devoted to case reports is increasing exponentially. Because of inconsistent and incomplete reporting of unusual cases, the Ca se Re port (CARE) reporting guidelines have been developed, analogous to CONSORT and STROBE guidelines. The CARE guidelines have a 13-point checklist for necessary elements of a case report. Similarly, the S urgical Ca se Re port (SCARE) guidelines were developed by consensus for surgical case reports. The effect of these new guidelines will need evaluation in the years ahead. Suggestions have been made for standardising the peer review of submitted case reports as well.
Case-series report
A case series aggregates individual cases in one report. It is defined as ‘a collection of subjects (usually patients) with common characteristics used to describe some clinical, pathophysiological, or operational aspect of a disease, treatment, exposure, or diagnostic procedure. Some are similar to the larger case reports and share their virtues’. Sometimes, the appearance of several similar cases in a short period heralds an epidemic. For example, a cluster of homosexual men in Los Angeles with a similar clinical syndrome alerted the medical community to the AIDS epidemic in North America. Whereas a report of a single unusual case might not trigger further investigation, a case series of several unusual cases (in excess of what might typically be expected) adds to the concern. A convenient feature of case-series reports is that they can constitute the case group for a case-control study, which can then explore hunches about causes of disease.
The distinction between case-series reports and cohort studies is murky. Dekkers et al. and Esene et al. have suggested that a case series samples participants based on an outcome (with or without regard to exposure); absolute risks cannot be determined. A cohort study chooses participants based on exposure , follows them to outcome, and calculates absolute risks. However, in their view, inclusion of a comparison group is not essential to be deemed a cohort study. Others continue to advocate the definitions we use in this book. Analogous to the SCARE guidelines described previously, the P referred R eporting O f C as e S eries in S urgery (PROCESS) guidelines have been promulgated for case-series reports in the surgical disciplines.
Cross-sectional (prevalence) studies
Prevalence studies describe the health of populations. The definition is ‘a study that examines the relationship between disease (or other health outcomes) and other variables of interest as they exist in a defined population at one particular time’. Synonyms include ‘prevalence study’ and ‘disease-frequency survey’. Cross-sectional studies can be considered a snapshot of the population at one moment of time; prevalence is the measure, as opposed to incidence. For example, in the United States, periodic surveys of the health status of the population are done by the federal government (e.g., the Health Interview Survey and the Health and Nutrition Examination Survey). Analogous to the decennial census, these studies provide an assessment of the population at a particular time.
Prevalence studies can be done in smaller populations as well. For example, a survey done in a Puerto Rican pharmaceutical factory indicated a high prevalence of gynaecomastia among employees ( Fig. 2.2 ). This finding led to the hunch that exposure to ambient oestrogen dust in the plant might be the cause; serum concentrations of oestrogen lent support to the hypothesis. After improvements in dust control in the factory, the epidemic disappeared. Gynaecomastia can be either endogenous from abnormal hormonal status or exogenous from drugs or herbs.
Although generally distinguished from cohort and case-control studies, the cross-sectional study can be thought of as a hybrid, the case-control analogue of a population cohort study. Because both exposure and outcome are ascertained at the same time (the defining feature of a cross-sectional study), costs are small and loss to follow-up is not a problem. However, because exposure and outcome are identified at one time point, the temporal sequence is often impossible to work out. An exception would be long-standing exposures, such as sex or blood type, which unquestionably preceded the outcome.
Surveillance
Surveillance is another important type of descriptive study. Surveillance can be thought of as watchfulness over a community. A more formal definition is ‘systematic and continuous collection, analysis, and interpretation of data, closely integrated with the timely and coherent dissemination of the results and assessment to those who have the right to know so that action can be taken’. The key feature here is feedback, as in a servomechanism. Prevention and control of the problem are fundamental parts of the feedback loop.
Surveillance can be either active or passive. Passive surveillance relies on data generally gathered through traditional channels, such as death certificates. By contrast, active surveillance searches for cases. Active surveillance can improve sensitivity. Investigators in the Philippines compared the incidence rate of dengue fever in a prospective seroepidemiological cohort study (active surveillance) with the incidence rate derived from the city health department (passive surveillance). The cumulative incidence rate was five times higher with active surveillance.
Active surveillance can improve specificity as well. For example, septic transfusion reactions remain a problem in hospitals. Active surveillance by culturing aliquots of transfused platelets over 7 years found a prevalence of bacterially contaminated products of 389/10 6 (20 of 51,440 units). Five neutropenic patients had septic transfusion reactions, and none was identified through passive surveillance. In contrast, 284 septic transfusion reactions were reported through passive surveillance, but none had received contaminated platelets. Passive surveillance had poor sensitivity and specificity.
Epidemiological surveillance has made important contributions to health, but none more impressive than smallpox eradication. Surveillance and containment were in part responsible for the elimination of smallpox from the world, an extraordinary public-health achievement. Without a nonhuman vector, the virus died out. Surveillance indicated that by 2014, four of six World Health Organization regions had been declared free of polio, and work continues towards its elimination. Rinderpest, a disease of cattle, was declared eradicated in 2011, and dracunculiasis (guinea worm disease) may be the next important disease to be wiped off the planet.
Ecological correlational studies
Correlational studies look for associations between exposures and outcomes in populations rather than in individuals. Because the data might already have been collected, correlational studies can be a convenient initial search for hypotheses. The measure of association between exposure and outcome is the correlation coefficient r , which indicates how linear is the relation between exposure and outcome. US counties at high elevation have lower rates of heart disease than do counties at low altitude. What this means is unclear. Another ecological study found no correlation between statin use and coronary mortality in Western Europe. In contrast, an inverse relationship between oral contraceptive use and ovarian cancer incidence has been documented in several countries, which is plausible, based on analytic study results.
Correlational studies have important limitations (i.e., the inability to link exposure to outcome in individuals and to control for confounding, a mixing or blurring of effects). A particular trap of ecological studies is an error in logic termed ‘ecological fallacy’, ‘an erroneous inference that may occur because an association observed between variables on an aggregate level does not necessarily represent or reflect the association that exists at an individual level…’. For example, an ecological correlation study compared nighttime light levels (judged by satellite imaging) and breast-cancer rates by census tract in Connecticut. Cancer rates came from the state tumour registry. The report implied causality in its results that ‘…support the possibility that electric light at night accounts for a portion of the breast cancer burden in high-risk societies’. Really? Before turning off street lights to protect women’s breasts, better evidence will be required. Like case reports and case-series reports, ecological studies can generate hypotheses but not definitive answers.
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