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CAT guide: PICOs

How to make a CAT (Critically Appraised Topic), a short summary of the best available evidence, created to answer a specific clinical question.

What is a PICO(S)?






Patient or Problem




Study type

The PICO(S)-method helps to focus a clinical question.

PICO(S) is an acronym:


A PICO(S) can be used for  all categories of clinical questions:  therapy, prognosis, risk/etiology, diagnosis. The key concepts in the examples  on this page are bold (the key  concepts are relevant for the search strategy)

Example 1 Therapy

Example 1A

P = adults with a common cold

I =  vitamin c pills

C = placebo pills

O = duration or intensity of the cold

S = randomized controlled trial


Example 1B 

P = adolescents with obsessive compulsive disorder

I =  cognitive behaviour therapy combined with medication (SSRIs)

C = medication-only (SSRIs)

O = daily functioning, performance at school, complaints/symptoms

S = randomized controlled trial


Example 2 Prognosis

Explanation: note that a prognostic factor, like depression in this example, cannot be randomized. The study design  is observational.  A prognostic factor can be a psychologic or physical factor, or a “circumstance” that is present.

P = cancer patients

I =  depression (as predictor)

C =  absence of depression

O =  disease progression, mortality

S =  observational study (cohort, follow-up)

Example 3 Risk/etiology

Explanation:  Note that randomization is not ethical in case of risk factors. The study design is observational. 

P =  babies, infants

I =  cosleeping with parents, bedsharing (as risk factor)

C =  sleeping sperately

O =  sudden death, SIDS, cot death

S =  observational (case-control) 

Example 4 Diagnosis

P =  (patients suspected of)  ceuliac disease

I =  serological test

C =  small bowel biopsy (i.e. the gold standard)

O =  diagnostic accuracy, sensitivity and specificity

S =  diagnostic study


Sjoukje van der Werf's picture
Sjoukje van der Werf
+31 (0)50 361 6197/ 6200
Subjects:- Medicine

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