Types of Research Studies

For medical research, there are different types of peer reviewed research.

Case Series and Case Reports
Case Control Studies
Cohort Studies
Randomized Controlled Studies
Double-Blind Studies
Meta-Analysis
Systematic Review

Identifying how the research study was designed will help you understand the kind of information they collected and what kind of evidence this research study offers.

Case Series and Case Reports

These are reports on the treatment of patient(s) with the same condition:

Case series/reports are used to illustrate an aspect of a condition.
Example: You have a patient that has a condition that you are unfamiliar with. You would search for case reports to assist on a diagnosis or decide on a direction of treatment.
Case series/reports have no control group (one to compare outcomes).
The benefits of case series/reports are that they are easy to understand and can be written up in a very short period of time.

Case Control Studies

Patients who have a condition are compared with people who do not.

Case control studies are generally designed to estimate the odds of developing a condition. They can determine if there is an association between a condition and risk factors.
Example: A study in which colon cancer patients are asked what kinds of food they have eaten and the answers are compared with a selected control group.
Case control studies are less reliable than either randomized controlled trials or cohort studies.

Cohort Studies

Also called longitudinal studies, these involve a population who presently have a certain condition and/or receive a particular treatment that are followed over time and compared with another group who are not affected by the condition or do not receive a particular treatment.

Cohort studies are used to establish causation of a condition or to evaluate the impact of treatment.
Example: One of the more well-know examples of a cohort study is the Framingham Heart Study, which followed generations of residents of Framingham, Massachusetts.
Cohort studies are not as reliable as randomized controlled studies, since the two groups may differ in ways other than the condition or treatment under study.

Randomized Controlled Studies

This is a study in which 1) There are two groups, one treatment group and one control group. The treatment group receives the treatment under investigation, and the control group receives either no treatment (placebo) or standard treatment. 2) Patients are randomly assigned to all groups.

Randomized controlled trials are considered the “gold standard” in medical research. They lend themselves best to answering questions about the effectiveness of different treatments.
Randomization helps avoid the bias that a physician might be subject to. It also increases the probability that differences between the groups can be attributed to the treatment under study.
Having a control group allows for a comparison of treatments – e.g., treatment A produced favorable results 56% of the time versus treatment B which only produced favorable results 25% of the time.
There are topics on which randomized controlled studies cannot be done for ethical reasons: For instance, if patients were asked to undertake harmful experiences (like smoking) or denied any treatment beyond a placebo when there are known treatments available.

Double-Blind Studies

This is a type of randomized controlled study in which neither the physician nor the patient knows which of several possible treatments the patient is receiving.

Example: Studies of treatments that consist essentially of taking pills are very easy to do double blind – the patient takes one of two pills of identical size, shape, and color, and neither the patient nor the physician needs to know which is which.
A double blind study is the most rigorous clinical research design because, in addition to the randomization of subjects, which reduces the risk of bias, it can eliminate or minimize the placebo effect which is a further challenge to the validity of a study.

Meta-Analysis

A meta-analysis is a systematic way to combine data from many studies, usually from randomized controlled studies, and arrive at a pooled estimate of treatment effectiveness:

Meta-analysis can also combine data from case control and cohort studies. Combining the data from multiple studies increases sample size and allows for analyses that would not otherwise be possible.

Systematic Review

A systematic review is a comprehensive survey of a topic that finds all relevant studies of the highest level of evidence, assesses each study, and synthesizes the finding. Systematic reviews present a balanced and impartial summary of the findings and consider any flaws in the evidence.

A systematic review is different from a literature review in that it is more rigorous and attempts to reduce bias by following a formal process:

Presenting a clearly formulated research question & an explicit methodology for assessing research.
Reviewing relevant published & unpublished (conferences, company reports, “file drawer reports”, etc.) literature.
Assessing the quality of the studies, the impact of bias, and the applicability of the findings.

A systematic review is different from a meta-analysis in that it takes a qualitative view and looks at the whole picture, while a meta-analysis takes a quantitative view and looks specifically at statistical results.

Types of Research Studies was adapted with permission from University of Minnesota Libraries.