The study type that you performed is usually mentioned in your article in the title, the abstract, the objectives of your study (in the introduction), and in the materials and methods section. Most researchers decide on or identify the study type of their research before starting their project as they describe the study in grant applications, clinicial trial registration, the application to an ethical committee, and other official documents. But I have noticed that some of my clients struggle to identify the correct description of the type of study they did. Here I describe the essential concepts of various study types and the level of evidence that can be gathered with these studies.

selecting study type may be difficult
there are many study types to choose from (image by Janusz Baścik for iStock)

Most common study types

Surveys and questionnaires:

In surveys, all data are self-reported by the participants. The investigators do not: check any data in medical records, perform any measurements, or provide any intervention. A survey may of course also be just part of a larger study. Surveys gather data to describe for instance: the demographics of a group;  the health status of a group of people at a particular time;  the use of medical services; or the knowledge, beliefs, and attitudes of people regarding health practices.

Cohort Study (Prospective Observational Study):

Cohort studies observe groups of individuals before they develop a disease or a particular outcome, and follow them over time to observe the outcome. This is a study without intervention.

Case-Control Study (Retrospective Study):

Case-control studies begin with the outcomes and do not follow people over time. Researchers select people with a particular result (the cases) or lack thereof (the controls) and check their medical records to determine what different experiences they had.

Cross-sectional study:

When exposure and outcome are determined simultaneously. Such as measuring a blood marker in a group of people, determining their weight, etc, all at a specific moment in time without looking into their past or following them up in the future.

Randomized Clinical Trials (RCTs):

RCTs follow two (or more) groups of people over time to see who achieves a particular result. In this case, the researchers randomize the people to the groups.  Each group then receives a different intervention. When the study ends, the researchers evaluate the outcomes of the different interventions and calculate the risk of one group developing a particular result compared to another.

Case Reports and Case Series:

A report on a patient or a series of patients with an outcome of interest. No control group is involved and it is usually done retrospectively. 

Help in selecting the correct study type

If you need help to determine the correct study type you could also first follow the EQUATOR decision tree to identify the correct guideline here. And then look back at this list what type of study the guideline describes. Many medical journals will ask you to follow these particular guidelines, and fill in the corresponding statement, so you will need to find the correct guideline anyway. You may also need to provide a flowchart, you can find tips on how to generate one here.

If you want to learn more about study design terminology this is a source  (look at the right column).

Level of evidence based on study type

For some medical journals the authors also need to indicate which level of evidence is achieved by the study. For evidence-based medicine a high level of evidence is of course required and articles with a higher level of evidence undoubtedly get a higher position on the editor`s list of priorities. In the below Table these levels are indicated for prognostic studies. There are many tables for different types of studies, this table gives a general indication of the levels of evidence obtained. Check with the guidelines of a specific journal to choose the correct category.

level of evidence for various study types
Level of evidence, based on an article by Song and Chung on observational studies