There are several different schemes for classifying study designs. We have adopted one that divides studies into those in which the subjects were merely observed, sometimes called observational studies, and those in which some intervention was performed, generally called experiments.
Observational studies are of four main types: case-series, case-control, cross-sectional, and cohort studies. When certain characteristics of a group (or series) of patients (or cases) are described in a published report, the result is called a case-series study; it is the simplest design in which the author describes some interesting or intriguing observations that occurred for a small number of patients.
Case-control, cross-sectional, and cohort studies are defined by the period of time the study covers and by the direction or focus of the research question. Cohort and case-control studies generally involve an extended period of time defined by the point when the study begins and the point when it ends; some process occurs, and a certain amount of time is required to assess it. For this reason, both cohort and case-control studies are sometimes also called longitudinal studies. The major difference between them is the direction of the inquiry or the focus of the research question: Cohort studies are forward-looking, from a risk factor to an outcome, whereas case-control studies are backward-looking, from an outcome to risk factors. The cross-sectional study analyzes data collected on a group of subjects at one time.
- Case-Series Studies
A case-series report is a simple descriptive account of interesting characteristics observed in a group of patients. Case-series reports generally involve patients seen over a relatively short time. Generally case-series studies do not include control subject, persons who do not have the disease or condition being described. Generally investigators would not include case-series in a list of types of studies because they are generally not planned studies and do not involve any research hypotheses.
- Case-Control Studies
Figure 1 Case-Control Stuides
Case-control studies begin with the absence or presence of an outcome and then look backward in time to try to detect possible causes or risk factors that may have been suggested in a case-series report. The cases in case-control studies are individuals selected on the basis of some disease or outcome; the controls are individuals without the disease or outcome. The history or previous events of both cases and controls are analyzed in an attempt to identify a characteristic or risk factor present in the cases’ histories but not in the controls’ histories. The Figure 1 on the left illustrates that subjects in the study are chosen at the onset of the study after they are known to be either cases with the disease or outcome (square) or controls without the disease or outcome (diamond). The histories of cases and controls are examined over a previous period to detect the presence (shaded areas) or absence (unshaded areas) of predisposing characteristics or risk factors, or, if the disease is infectious, whether the subject has been exposed to the presumed infectious agent. In case-control designs, the nature of the inquiry is backward in time, as indicated by the arrow pointing backward in figure 1 to illustrate the backward, or retrospective, nature of the research process. We can characterize case-control studies as studies that ask “what happened?” In fact, they are sometimes called retrospective studies because of the direction of inquiry. Case-control studies are longitudinal as well because the inquiry covers a period of time.
Investigators sometimes use matching to associate controls with cases on characteristics such as age and sex. If an investigator feels that such characteristics are so important that an imbalance between the two groups of patients would affect any conclusions, he or she should employ matching. This process ensures that both groups will be similar with respect to important characteristics that may otherwise cloud or confound the conclusions.
- Cross-Sectional Studies
Cross-sectional studies also are known as surveys, epidemiological studies, and prevalence studies. Cross-sectional studies analyse data collected on a group of subjects at one time rather than over a period of time. Cross-sectional studies are designed to determine “What is happening?” right now. Subjects are selected and information is obtained in a short period of time.
Figure 2 Cross-Sectional Studies
- Cohort Studies
A cohort is a group of people who have something in common and who remain part of a group over an extended time. In medicine, the subjects in cohort studies are selected by some defining characteristic (or characteristics) suspected of being a precursor to or risk factor for a disease or health effect. Cohort studies ask the question “What will happen?” and thus, the direction in cohort studies is forward in time. For cohort studies, researcher select subjects at the onset of the study and then determine whether they have the risk factor or have been exposed. All subjects are followed over a certain period of time to observe the effect of the risk factor or exposure. Because the events of interest transpire after the study is begun, these studies are sometimes called prospective studies.
Many cohort studies are prospective; that is, they begin at a specific time, the presence or absence of the risk factor is determined, and then information about the outcome of interest is collected at some future time. However, one also can undertake a cohort study by using information collected in the past and kept in records or files, which we call it historical cohort studies. This approach to a study is possible if the records on follow-up are complete and adequately detailed and if the investigators can ascertain the current status of the patients.
Some investigators also call this type of study a retrospective cohort study because historical information is used; that is the events being evaluated actually occurred before the onset of the study. The direction of the inquiry is till forward in time.
Figure 3 Cohort Studies