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Critical Appraisal

Information and resources on how to assess the quality and relevance of articles

PICO is a tool to assist in forming focused clinical questions. It is an acronym of:

Population - Intervention - Comparison - Outcome

By breaking down your question in this way it becomes easier to critique what you are reading.


Population, pathology, patient Intervention, exposure, prognosis Comparison Outcome

What are the characteristics of the population?

What is the disease or condition?

How would you describe them?

How were participants recruited / selected?

What is your treatment?

What are you observing?

What are you comparing against?

Drug, surgery, placebo, no intervention, usual care?

What are you trying to improve or measure?

What outcomes are being assessed – objective / subjective /surrogate? 

Positive outcomes - healing time, reduced infection

Negative outcomes - adverse effects, mortality


Example. What is the evidence for the use of intrathecal delivery of baclofen to control spasticity in cerebral palsy?

Cerebral palsy

Intrathecal pumps


Usual care

Muscle spasticity

You do not need to use all elements of PICO. Not all questions suit the PICO format, so there are other versions you can use.

Social interventions use PICOC:          Population - Intervention - Comparison - Outcome - Context

Restricting study types, use PICOS:    Population - Intervention - Comparison - Outcome - Study

Therapy questions use PICOT:            Population - Intervention - Comparison - Outcome - Time

Other breakdowns include:

SPIDER for qualitative research:         Sample - Phenomenon of Interest - Design - Evaluation - Research type 



  • The important part is breaking down your question to sensible concepts - you do not need to use all four parts of PICO.
  • When searching, distinguish between what you search for, and what you screen for.
    • Sometimes it is better to search for just a couple of your concepts (ep population and intervention), then screen the results for your other concept of interest (eg outcome). this is because....
  • outcomes are often not well described in abstracts. Remember, when searching in databases such as Medline you are not searching the full text of the article, but only the title, abstract and indexing.
    • Searching the word 'Outcome' will return all the abstracts with a study outcome section, not just clinical or therapeutic outcome.
  • comparisons are also not well described. This is especially true in observational studies.
    • Comparisons as a search term work best in comparison types of study, such as RCT's.

More PICO resources

BMJ - How to clarify a clinical question.