Hypothesis Formulation – Characteristics of Person, Place, and Time
Descriptive epidemiology searches for patterns by examining characteristics of person, place, & time. These characteristics are carefully considered when a disease outbreak occurs, because they provide important clues regarding the source of the outbreak.
Hypotheses about the determinants of disease arise from considering the characteristics of person, place, and time and looking for differences, similarities, and correlations. Consider the following examples:
- Differences: if the frequency of disease differs in two circumstances, it may be caused by a factor that differs between the two circumstances. For example, there was a substantial difference in the incidence of stomach cancer in Japan & the US. There are also substantial differences in genetics and diet. Perhaps these factors are related to stomach cancer.
- Similarities: if a high frequency of disease is found in several different circumstances & one can identify a common factor, then the common factor may be responsible. Example: AIDS in IV drug users, recipients of transfusions, & hemophiliacs suggests the possibility that HIV can be transmitted via blood or blood products.
- Correlations: If the frequency of disease varies in relation to some factor, then that factor may be a cause of the disease. Example: differences in coronary heart disease vary with cigarettes consumption.
Descriptive epidemiology provides a way of organizing and analyzing data on health and disease in order to understand variations in disease frequency geographically and over time and how disease varies among people based on a host of personal characteristics (person, place, and time). Epidemiology had its origins in the desire to understand the determinants of acute infectious diseases, but its methods and applicability have expanded to include chronic diseases as well.
Descriptive Epidemiology for Infectious Disease Outbreaks
Outbreaks generally come to the attention of state or local health departments in one of two ways:
- Astute individuals (citizens, physicians, nurses, laboratory workers) will sometimes notice cases of disease occurring close together with respect to time and/or location or they will notice several individuals with unusual features of disease and report them to health authorities.
- Public health surveillance systems collect data on 'reportable diseases'. Requirements for reporting infectious diseases in Massachusetts are described in 105 CMR 300.000 (Link to Reportable Diseases, Surveillance, and Isolation and Quarantine Requirements).
Clues About the Source of an Outbreak of Infectious Disease
When an outbreak occurs, one of the first things that should be considered is what is known about that particular disease. How can the disease be transmitted? In what settings is it commonly found? What is the incubation period? There are many good summaries available online. For example, Massachusetts DPH provides this link to a PDF fact sheet for Hepatitis A, which provide a very succinct summary. With this background information in mind, the initial task is to begin to characterize the cases in terms of personal characteristics, location, and time (when did they become ill and where might they have been exposed given the incubation period for that disease. In sense, we are looking for the common element that explains why all of these people became ill. What do they have in common?
Information about the cases is typically recorded in a "line listing," a grid on which information for each case is summarized with a separate column for each variable. Demographic information is always relevant, e.g., age, sex, and address, because they are often the characteristics most strongly related to exposure and to the risk of disease. In the beginning of an investigation a small number of cases will be interviewed to look for some common link. These are referred to as "hypothesis-generating interviews." Depending on the means by which the disease is generally transmitted, the investigator might also want to know about other personal characteristics, such as travel, occupation, leisure activities, use of medications, tobacco, drugs. What did these victims have in common? Where did they do their grocery shopping? What restaurants had they gone to in the past month or so? Had they traveled? Had they been exposed to other people who had been ill? Other characteristics will be more specific to the disease under investigation and the setting of the outbreak. For example, if you were investigating an outbreak of hepatitis B, you should consider the usual high-risk exposures for that infection, such as intravenous drug use, sexual contacts, and health care employment. Of course, with an outbreak of foodborne illness (such as hepatitis A), it would be important to ask many questions about possible food exposures. Where do you generally eat your meals? Do you ever eat at restaurants or obtain foods from sources outside the home? Hypothesis generating interviews may quickly reveal some commonalities that provide clues about the possible sources.
Assessment of an outbreak by place provides information on the geographic extent of a problem and may also show clusters or patterns that provide clues to the identity and origins of the problem. A simple and useful technique for looking at geographic patterns is to plot, on a "spot map" of the area, where the affected people live, work, or may have been exposed. A spot map of cases may show clusters or patterns that reflect water supplies, wind currents, or proximity to a restaurant or grocery store.
In 1854 there was an epidemic of cholera in the Broad Street area of London. John Snow determined the residence or place of business of the victims and plotted them on a street map (the stacked black disks on the map below). He noted that the cases were clustered around the Broad Street community pump. It was also noteworthy that there were large numbers of workers in a local workhouse and a brewery, but none of these workers were affected - the workhouse and brewery each had their own well.
On a spot map within a hospital, nursing home, or other such facility, clustering usually indicates either a focal source or person-to-person spread, while the scattering of cases throughout a facility is more consistent with a common source such as a dining hall. In studying an outbreak of surgical wound infections in a hospital, we might plot cases by operating room, recovery room, and ward room to look for clustering.
- Link to more on the outbreak of cholera in the Broad Street area of London
- Link to an enlarged version of Snow's spot map
When investigating the source of an outbreak of infectious disease, Investigators record the date of onset of disease for each of the victims and then plot the onset of new cases over time to create what is referred to as an epidemic curve. The epidemic curve for an outbreak of hepatitis A is shown in the illustration below. Begriming in late April, the number of new cases rises to a peak of twelve new cases reported on May 12, and then the number of new cases gradually drops back to zero by May 21. Knowing that the incubation period for hepatitis A averages about 28-30 days, the investigators concluded that this was a point source epidemic because the cluster of new cases all occurred within the span of a single incubation period (see explanation on the next page). This, in conjunction with other information, provided important clues that helped shape their hypotheses about the source of the outbreak.
Video Summary: Person, Place, and Time (10:42)