Three of the most important study designs in modern epidemiology are ecologic studies, cross-sectional studies, and case-control studies. Briefly explain each of these study designs. Also, for each design, present at least one example of a research scenario in which the design would be appropriate.
Ecologic studies-In ecological studies the unit of observation is the population or community. Disease rates and exposures are measured in each of a series of populations and their relation is examined. Often the information about disease and exposure is abstracted from published statistics and therefore does not require expensive or time-consuming data collection. The populations compared may be defined in various ways. An example would be to look at geographical correlations between disease incidences and mortality rates and the prevalence of risk factors. The mortality rate of people dying of measles in the present day in Houston and compare it to the mortality rate of the measles mortality rate 50 years earlier.
In a cross-sectional study, data are collected on the whole study population at a single point in time to examine the relationship between disease (or other health related state) and other variables of interest.
Cross-sectional studies therefore provide a snapshot of the frequency of a disease or other health related characteristics in a population at a given point in time. This methodology can be used to assess the burden of disease or health needs of a population, for example, and is therefore particularly useful in informing the planning and allocation of health resources. An example of a research scenario would be the random sample of children in the urban school elementary schools prevalence of excessive absenteeism due to poor compliance of asthma treatment.
Case Control Studies- By definition, a case-control study is always retrospective because it starts with an outcome then traces back to investigate exposures. When the subjects are enrolled in their respective groups, the outcome of each subject is already known by the investigator. This, and not the fact that the investigator usually makes use of previously collected data, is what makes case-control studies ‘retrospective. An example of research pertaining to this would be a situation in which a large number of cases of post-operative endophthalmitis have occurred in a few weeks. The case group would consist of all those patients at the hospital who developed post-operative endophthalmitis during a pre-defined period.