Research Roadmap

Study design

Study design is not a choice; it should match the research question to ensure the most unbiased answers. Selecting an appropriate study design for the research question is critical for effectively addressing the specific aims of your study. The study design is determined by the study objectives and the resources vailable. A helpful reference for understanding the taxonomy of clinical study designs is "An overview of clinical research: the lay of the land" by Grimes DA and Schulz KF (Lancet 2002;359, 57-61). Note that it is not exhaustive, but it covers the most basic clinical study designs.

If you have any questions about selecting a suitable research design, you can request a consultation with the department of Epidemiology and Data Science (EDS) via the EDS-Consult Service Portal.

Sample size

The planned number of study participants (sample size) is a key aspect of study design, budgeting, and feasibility.

For scientific and ethical reasons, the sample size needs to be considered carefully to ensure the study can provide a reliable answer to the research question(s). 

The sample size is usually determined using a formal sample size calculation. All assumptions supporting the calculation - both clinical and statistical - should be justified and reported in the research protocol. 

The sample size should be large enough to have a high probability (power) to detect a clinically meaningful effect or difference, if one exists. Sample size calculations are generally based on one primary outcome. However, it may also be worthwhile to consider and report the study's power (given the proposed sample size) for other important outcomes or analyses. If the sample size is not derived statistically, this should be explicitly stated, along with a rationale - for example, in the case of exploratory pilot studies or pragmatic considerations for trials involving rare diseases.  

In cases where essential information for a sample size calculation is unavailable - such as the expected effect size for a clinically meaningful effect/difference - a pilot study can facilitate these estimations. 

It is important to note that these basic principles for a formal sample size calculation primarily apply to (therapeutic) interventional studies, such as randomized clinical trials. For other types of research, such as prognostic studies, a sample size calculation is less straightforward.

If you need assistance with determining your sample size, you can request a consultation with EDS via the EDS-Consult Service Portal.        

Note that the Medical Research Ethics Committee (MREC) and most study grant applications (e.g. ZonMw) ask for a sample size calculation in the approval process.

To get an idea of the feasibility of your study; make a detailed estimate of the number of eligible participants you think you will be able to find:

• How common is this clinical picture?
• How many patients with such symptoms are seen each year at each participating hospital?
• How many patients would meet your in- and exclusion criteria?
• How many are likely to consent to participate in your study?
• Are there competing studies ongoing?
• What dropout rate do you expect during the study?

If you need more insight into the number of patients that meet your in- and exclusion criteria, you can request this information from the Research Data Platform (RDP) of the Department of Business Intelligence.