Descriptive Epidemiology: Study Types & Designs

• The presentation discusses Descriptive Epidemiology, specifically study types and designs

Epidemiology Study Types

• Descriptive studies monitor public health, evaluate intervention programs, and generate hypotheses about disease causes
• Analytic/Scientific studies evaluate hypotheses about disease causes and intervention program success

Descriptive Studies

• Identify and count disease cases in populations by person, place, and time by conducting simple studies
• Case Reports & Series, Cross-sectional Studies, and Ecologic Studies are examples of simple studies

Analytic Study Types

• Compare groups and systematically determine if there is an association
• Clinical Trial, Experimental Study, Case-Control Study, and Cohort Study are examples of analytic studies

Person

• Encompasses personal characteristics such as age, gender, race, ethnicity, socioeconomic status, occupation, religion, and marital status

Place

• Includes geographical units (countries or states) and natural geographic features (mountains or rivers)
• Place is characterized by physical, biological, and social environments
• Malaria occurs where facets of the environment are conducive to the life cycle of Anopheles mosquito (favorable temperature, adequate humidity, moderate rainfall, presence of standing or flowing water)
• Migrant studies investigate the effect of place on disease occurrence, comparing disease rates among natives, immigrants, and the adopted country's natives

Time

• Involves changes in disease and death rates over calendar time
• Time includes short-term trends for infectious diseases and long-term trends for chronic diseases
• Other temporal trends include periodic fluctuations, like seasonal variations in influenza infections
• Temporal trends may be due to changes in exposure to causal agents and/or susceptibility to disease

Case Reports and Series

• A case report provides a detailed report on one patient with a new or unusual symptom or problem
• A case series provides a detailed report on a group of patients with the same symptom or problem
• These are useful for recognizing new diseases, detecting drug side effects, providing insight into disease mechanisms, and helping develop hypotheses
• Their main limitation is the lack of an explicit comparison group
• From October 1980 to May 1981, five young, active gay men were treated for biopsy-confirmed Pneumocystis carinii pneumonia at three different hospitals in LA; this was a case series

Ecological Studies

• Studies examine rates of disease in relation to a population-level factor
• The unit of observation is the group, rather than the individual
• Exposure and outcome data are collected at the group level
• Example: do smokers have higher death rates from coronary heart disease than non-smokers?
• Consider US states with higher smoking rates and whether they have higher death rates from coronary heart disease than states with lower smoking rates
• Exposure = per capita average cigarette sales in each state
• Outcome = coronary heart disease mortality "rates" in each state

Ecological Fallacy or Bias

• Exposure is measured as an average for a population, so there is no real link between exposure and disease
• A focus is on individual-level effects
• Strong associations were seen in previous examples, but it may not be certain that people who smoked were the ones who died from coronary heart disease

Limitations of Ecological Studies

• The group-level association may not transfer to the individual level
• Also cannot adjust for confounding factors that may account for the observed association
• Complex relationships can be masked

Ecological Studies Strengths

• Inexpensive and fast
• Conducted on available data and good for the early stages of knowledge
• They allow for a wider range of exposures and may be useful for studying ecologic relationships
• Analysis is easy using a correlation coefficient or linear regression

Cross-Sectional Studies and Surveys

• They examine the relationship between an exposure and disease at a single point in time
• They measure exposure prevalence in relation to disease prevalence
• Many government surveys are cross-sectional

Cross-Sectional Surveys

• Survey of Coronary Heart Disease (CHD) Among Farm Owners by Occupational Physical Activity
• Farm owners who are not physically active have 4.75 times the prevalence of CHD as farmer owners who are physically active

Limitations of Cross-Sectional Studies

• Difficulty inferring temporal sequence of exposure & outcome ("Which came first?")
• These are problematic when exposure is a changeable characteristic (e.g., smoking, drinking, physical activity)
• These are OK for immutable characteristics (e.g., genetic traits, blood type), measures of long-term exposure (e.g., lead in bones), and historical exposure (e.g., activity level over the past 30 years)
• Its limitations include reliance on prevalent cases and the inability to determine incidence and duration

Strengths of Cross-Sectional Studies

• Relatively quick and inexpensive
• Highly generalizable if based on the general population
• Problem with temporal inference can be avoided for inalterable, long-term, and historical exposures

Summary of Descriptive Studies

• Case Report/Series reports on one or more patients with the same symptom/problem
• Ecological Studies study rates of disease in relation to population-level factors
• Cross-Sectional Studies examine exposure-disease relationships at a single time point, comparing exposure prevalence vs. disease prevalence
• Case Report/Series is simple, fast, and provides valuable insights
• Ecological Studies are simple, fast, inexpensive, and can be interested in ecological exposures
• Cross-Sectional Studies are quick and inexpensive
• Case Report/Series have no explicit comparison group
• Ecological studies have no information on individuals and limited confounder information
• Cross-Sectional Studies lack clear temporal relationships if exposure is changeable
• Tendency to identify long-duration cases