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Study Designs

An epidemiological study could be designed in several ways so as to collect new data. Two basic principles should always be followed: the study should be comparative and all potential causes of bias should be avoided. We can only be able to make judgments about association though comparisons. Also, Bias error can lead to erroneous conclusions about association and causation.

Four main classes of study type may be identified: descriptive studies, analytical studies, experimental studies and theoretical studies:

Descriptive studies
​​A descriptive study, also called a descriptive epidemiology, is to identify patterns or trends in a situation, but not the causal linkages among its different elements. Descriptive studies (such as a cross-sectional study) help in generating hypothesis on which further research may be based.

  • Cross-sectional studies
  • Screening
  • Ecological Studies
​​​Cross-section Studies

A cross-sectional study examines the relationship between disease (or other health related state) and other variables of interest as they exist in a defined population at a single point in time or over a short period of time (e.g. calendar year). Cross-sectional studies can be thought of as providing a snapshot of the frequency of a disease or other health related characteristics (e.g. exposure variables) in a population at a given point in time. Cross-sectional studies are used to assess the burden of disease or health needs of a population and are particularly useful in informing the planning and allocation of health resources.
​​​Screening

A screening is to find out suspected or potential patients from a superficially healthy population by using a test that is easy to perform and interpret, acceptable, accurate, reliable, sensitive and specific. A screening test result is not a diagnosis. A positive or suspected positive result from screening test indicates that the respondent should be designated to see a specialist for further diagnosis. Screening interventions are designed to identify disease in a community early, thus enabling earlier intervention and management in the hope to reduce mortality and suffering from a disease. Although screening may lead to an earlier diagnosis, not all screening tests have been shown to benefit the person being screened; over diagnosis, misdiagnosis, and creating a false sense of security are some potential adverse effects of screening. For these reasons, a test used in a screening program, especially for a disease with low incidence, must have good sensitivity in addition to acceptable specificity.
​​​Ecological Studies

A study in which at least one variable is measured at the group (not individual) level. Examples of group-level measures include the rate of cancer incidence, the mean level of hypertension, the average sunlight exposure at specific geographic location, or even the preventive service included in a health insurance plan. The occurrence of disease is compared between groups that have different levels of exposure, thus offering a comparison group for this study design.
Analytical Studies
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  • Case-control studies
  • Cohort studies
Case-control studies

​A case-control study is designed to help determine if an exposure is associated with an outcome (i.e., disease or condition of interest). In theory, the case-control study can be described simply. First, identify the cases (a group known to have the outcome) and the controls (a group known to be free of the outcome). Then, look back in time to learn which subjects in each group had the exposure(s), comparing the frequency of the exposure in the case group to the control group. 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’.
Cohort studies

​For research purposes, a cohort is any group of people who are linked in some way and followed over time. Researchers observe what happens to one group that’s been exposed to a particular variable — for example, the effect of company downsizing on the health of office workers. This group is then compared to a similar group that hasn’t been exposed to the variable.
Experimental Studies
An experiment or intervention study is a study in which a treatment, procedure, or program is intentionally introduced and a result or outcome is observed. ​​
True experiments have four elements: manipulation, control, random assignment and random selection. The most important of these elements are manipulation and control
  • Randomized control trial
  • Clinical trial
  • Community trial
Randomized control trial

​​The randomized controlled trial is one of the simplest but most powerful tools of research. In essence, the randomized controlled trial is a study in which people are allocated at random to receive one of several clinical interventions On most occasions, the term “intervention” refers to treatment, but it should be used in a much wider sense to include any clinical maneuver offered to study participants that may have an effect on their health status. Such clinical maneuvers include prevention strategies, screening programs, diagnostic tests, interventional procedures, the setting in which health care is provided, and educational models. Randomized controlled trials in radiology can play a major role in the assessment of screening programs, diagnostic tests, and procedures in interventional radiology. Randomized controlled trials are used to examine the effect of interventions on particular outcomes such as death or the recurrence of disease. Some consider randomized controlled trials to be the best of all research designs, or “the most powerful tool in modern clinical research”, mainly because the act of randomizing patients to receive or not receive the intervention ensures that, on average, all other possible causes are equal between the two groups. Thus, any significant differences between groups in the outcome event can be attributed to the intervention and not to some other unidentified factor. However, randomized controlled trials are not a panacea to answer all clinical questions; for example, the effect of a risk factor such as smoking cannot ethically be addressed with randomized controlled trials. Furthermore, in many situations randomized controlled trials are not feasible, necessary, appropriate, or even sufficient to help solve important problems. Randomized controlled trials are not appropriate for cancer screening, a situation in which the outcome is rare and frequently occurs only after a long delay. Thus, although the test for appraising the ultimate value of a diagnostic test may be a large well-designed randomized controlled trial that has patient outcomes as the end point, the trial should presumably be performed after other smaller studies have examined the predictive value of the test against some accepted standard.
Clinical trial

​​Clinical trials are research studies that test how well new medical approaches work in people. Each study answers scientific questions and tries to find better ways to prevent, screen for, diagnose, or treat a disease. Clinical trials may also compare a new treatment to a treatment that is already available. Every clinical trial has a protocol, or action plan, for conducting the trial. The plan describes what will be done in the study, how it will be conducted, and why each part of the study is necessary. Each study has its own rules about who can take part. Some studies need volunteers with a certain disease. Some need healthy people. Others want just men or just women.
Community trial

​​Community trials, also called community intervention studies, are (mostly preventive) experimental studies with whole communities (such as cities or states) as experimental units; that is, interventions are assigned to all members in each of a number of communities. These are to be distinguished from field trials where interventions are assigned to healthy individuals in a community and from clinical trials in which interventions are assigned to patients in a clinical setting. Except for the experimental unit, the conduct of controlled community trials follow the same procedures as controlled clinical trials, including the requirement of informed consent of the communities meeting the eligibility criteria (e.g., consent being given by city mayors or state governors), randomization to treatment and control groups, and follow-up and measurement of endpoints. In contrast to clinical trials, blinding and double blinding are not generally used in community trials.
Theoretical Studies
A theoretical study is also called mathematical model. It is a method of simulating real-life situations with mathematical equations to forecast their future behavior. Mathematical modeling uses tools such as decision-theory, queuing theory, and linear programming, and requires large amounts of number crunching.