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PubMed (EN): 1. Search question and scheme

Search scheme

A good research question consists of several elements that need to be present in your search results. To find as many articles as possible that can answer your research question you can also add relevant synonyms and related terms to your search. To structure a clinical search, methods such as PICO or DDO can be used. Of course it is also possible to identify key elements without these methods. 

A search scheme can help to structure your query. An empty scheme is provided below. In the log below you can also write down the choices you have made along the way. This can also help in explaining your search strategy. 

Below you can find an example research question that consists of several elements. These elements are also identified in the scheme below.

PICO, DDO and elements

PICO and DDO are acronyms that can help to structure a clinical search. Each letter stands for a different part of a research question. A PICO question consists of the following elements:

  • Patient:            Characteristics such as a disease or age category
  • Intervention:    Treatment, tool or method
  • Comparison:    Alternate intervention
  • Outcome:         Measurable result of the intervention(s)

The DDO-method is used for a more general clinical search. DDO consists of the following elements:

  • Domain:          Area of research
  • Determinant:  Key point of interest
  • Outcome:       Measurable result

For both methods not all the letters of the acronym need to be present, and some letters may be present several times. If for example a patient group consists of more than two key elements such as an age category and a disease then you can add both as separate elements. 

If your research question is not clinical you can identify key elements of your research question yourself. Usually between 3 and 5 elements results in a workable number of articles, depending on the subject, the number of terms per element and the used filters.


Search scheme

Example research question

In this example we will use the following research question:

How can C. elegans be used to study the effects of eating fruit on the ageing process?

Since this is not a clinical question we will identify the relevant elements ourselves:

E1: C. elegans

E2: eating

E3: fruit

E4: ageing

This will result in the search scheme below. The MeSH-terms will be added in the following step.

  Synonyms Related words MeSH-terms

Element 1:

C. elegans

Caenorhabditis Elegans




Element 2: 







Element 3: 


Plant aril



Element 4: 


Aging (American English)