Skip to main content
Ask a Librarian

EBM Resource Center: Ask the Question

Asking a Clinical Question

EBM imageOne of the basic skills required for practicing EBM is developing of well-built clinical questions. These questions need to be relevant to patients’ problems and phrased in ways that facilitate your acquisition of relevant and precise answers. Well-built clinical questions usually contain up to four elements. PICO is an acronym/pneumonic of these elements and it identifies and organizes the key aspects of a complex patient presentation: P=Patient or Population and Problem; I=Intervention or Indicator; C=Comparison or Control (not part of all questions); O=Outcome.

Adding the two T's (Type of Question, Type of Study) to the PICO framework addresses that different types of study designs are used to answer different types of questions. 

Two Cardinal Rules of EBM

  1. Not all evidence is created equal - A hierarchy of evidence guides clinical decision-making.
  2. Evidence alone is never enough - Competent physicians balance risks and benefits of management strategies in the context of patient values and preferences. 

What is PICO?


How would you describe a group of patients similar to yours? What are the most important characteristics of the patient?

What main intervention are you considering? What do you want to do with this patient?

What is the main alternative being considered, if any?

What are you trying to accomplish, measure, improve or affect?

Time Frame

Type of Question
Therapy / Diagnosis / Harm / Prognosis / Prevention

Type of Study
Systematic review / RCT / cohort study / case control




When to use PICO?

When to use the P.I.C.O. model...

Background questions concern general knowledge.  These types of questions generally have only 2 parts: A question root (who, what, when, where, how, why) and a disorder, test, treatment, or other aspect of health care.   Often these questions can best be answered by using a textbook or consulting a clinical database.

Foreground questions are specific knowledge questions that affect clinical decisions, including a broad range of biologic, psychological, and sociologic issues.  These are the questions that generally require a search of the primary medical literature and that are best suited to the PICO format.

Hierarchy of Evidence

The type of question will often dictate the best study design to address the question:

Types of Studies chart

Types of Studies


A quantitative method of combining the results of independent studies, which are drawn from the published literature, and synthesizing summaries and conclusions.

Systematic Review

A review which endeavors to consider all published and unpublished material on a specific question.  Studies that are judged methodologically sound are then combined quantitatively or qualitatively depending on their similarity.

Randomized Controlled Trial (RCT)

A  clinical trial involving one or more new treatments and at least one control treatment with specified outcome measures for evaluating the intervention.  The treatment may be a drug, device, or procedure. Controls are either placebo or an active treatment that is currently considered the "gold standard".  If patients are randomized via mathmatical techniques then the trial is designated as a randomized controlled trial.

Cohort Study

In cohort studies, groups of individuals, who are initially free of disease, are classified according to exposure or non-exposure to a risk factor and followed over time to determine the incidence of an outcome of interest.  In a prospective cohort study, the exposure information for the study subjects is collected at the start of the study and the new cases of disease are identified from that point on.  In a retrospective cohort study, the exposure status was measured in the past and disease identification has already begun. 

Case-control Study

Studies that start by identifying persons with and without a disease of interest (cases and controls, respectively) and then look back in time to find differences in exposure to risk factors. 

Cross-sectional Study

Studies in which the presence or absense of disease or other health-related variables are determined in each member of a population at one particular time. 

Experimental vs. Observational

An observational study is a study in which the investigator cannot control the assignment of treatment to subjects because the participants or conditionsare not being directly assigned by the researcher. 

  • Examines predetermined treatments, interventions, policies, and their effects
  • Four main types: case-seriescase-controlcross-sectional, and cohort studies

In an experimental study, the investigators directly manipulate or assign participants to different interventions or environments. 

  • Experimental studies that involve humans are called clinical trials. They fall into two categories: those with controls, and those without controls. 
    • Controlled trials - studies in which the experimental drug or procedure is compared with another drug or procedure
    • Uncontrolled trials - studies in which the investigators' experience with the experimental drug or procedure is described, but the treatment is not compared with another treatment

Experimental vs. Observational Studies with Examples (Iowa State University)

Formal Trials versus Observational Studies (Ravi Thadhani, Harvard Medical School)

Study Designs (Centre for Evidence Based Medicine, University of Oxford)

Learn about Clinical Studies (, National Institutes of Health)


Definitions taken from: Dawson B, Trapp R.G. (2004). Chapter 2. Study Designs in Medical Research. In Dawson B, Trapp R.G. (Eds), Basic & Clinical Biostatistics, 4eRetrieved September 15, 2014 from