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Levels of Evidence

Evidence-Based Practice, Levels of Evidence, and Clinical Questions

Types of Clinical Questions

There are two types of clinical questions;  background and foreground.

If you know what type of question you are working with, you can then determine the best resources or types of evidence to consult for answers.

Background questions ask for general knowledge about an illness, disease, condition, process or thing. These types of questions typically ask who, what, where, when, how & why about things like a disorder, test, or treatment, etc.

For example...

  • How overweight is a woman to be considered slightly obese?
  • What are the clinical manifestations of menopause?
  • What causes migraines?

A background question might be useful for structuring a general research paper on a topic.

Foreground questions ask for specific knowledge to inform clinical decisions. These questions typically concern a specific patient or particular population. Foreground questions tend to be more specific and complex compared to background questions. Quite often, foreground questions investigate comparisons, such as two drugs, two treatments, two diagnostic tests, etc. Foreground questions may be further categorized into one of 4 major types: treatment/therapy, diagnosis, prognosis, or etiology/harm.

For example:

  • Are female non-smokers with daily exposure to second-hand smoke over a period of ten years or greater more likely to develop breast cancer when compared with female non-smokers without daily exposure to second hand smoke?
  • Does dietary carbohydrate intake influence healthy weight maintenance (BMI <25) in patients who have family history of obesity (BMI >30)?

Foreground questions will typically be those that can be structured using a research framework and are the type of question most suited to many of your assignments at CGI, such as literature reviews or intervention proposals.  PICO is probably the most common research framework for foreground questions.

Digging Deeper on on Foreground Questions

Foreground questions typically fall into five different categories:

Type of Question You know you have this type of question if you are asking ... More Information
Therapy or Intervention What should I do to help my patient? Therapy questions explore potential interventions that could be used to treat or manage a patient's condition. They are used to evaluate the effectiveness of different medications, physical therapy, surgical procedures, lifestyle changes, etc.
Prognosis What will happen to my patient in the future?  Prognosis questions explore the likelihood of particular outcomes for patients with particular disorders. They are used to predict the patient's expected development and anticipate any future complications.
Harm or Etiology Why has this happened to my patient?  Harm/etiology questions explore the causes and likelihood of a health care problem. They are used to find the origin of a patient's condition so that decisions about their care can be made.
Diagnosis How should I determine if my patient has a particular condition? Diagnosis questions compare the accuracy and safety of diagnostic tests against the standard method. They are used to determine which test will be the most accurate in confirming or excluding a particular condition.
Prevention How can I prevent a specific outcome for my patient? Prevention questions explore ways to reduce the likelihood of particular condition or disease. They are used to reduce the chance of disease by identifying and modifying risk factors.

Many thanks to the Evidence-Based Practice Research Guide at University of South Australia for information for this chart.

Matching a Question to the Evidence

Knowing the type of foreground question you have can help you select the best study design or type of evidence to answer your question.

In general, you always want to look for the study design that will yield the highest level of evidence.

This pyramid chart from the Dahlgren Memorial Library at Georgetown University Medical Center matches the type of foreground question to the best type of evidence.

study designs and levels of evidence pyramid chart


Many thanks to the Evidence-Based Practice Resource Guide at Georgetown University Dahlgren library for the graphic and explanations of evidence below.

Definitions of Study Types

(From BMJ’s Clinical Evidence Glossary)

Meta-analysis: A statistical technique that summarizes the results of several studies in a single weighted estimate, in which more weight is given to results of studies with more events and sometimes to studies of higher quality.

Systematic Review: a review in which specified and appropriate methods have been used to identify, appraise, and summarize studies addressing a defined question. (It can, but need not, involve meta-analysis). In Clinical Evidence, the term systematic review refers to a systematic review of RCTs unless specified otherwise.

Randomized Controlled Trial: a trial in which participants are randomly assigned to two or more groups: at least one (the experimental group) receiving an intervention that is being tested and another (the comparison or control group) receiving an alternative treatment or placebo. This design allows assessment of the relative effects of interventions.

Controlled Clinical Trial: a trial in which participants are assigned to two or more different treatment groups. In Clinical Evidence, we use the term to refer to controlled trials in which treatment is assigned by a method other than random allocation. When the method of allocation is by random selection, the study is referred to as a randomized controlled trial (RCT). Non-randomized controlled trials are more likely to suffer from bias than RCTs.

Cohort Study: a non-experimental study design that follows a group of people (a cohort), and then looks at how events differ among people within the group. A study that examines a cohort, which differs in respect to exposure to some suspected risk factor (e.g. smoking), is useful for trying to ascertain whether exposure is likely to cause specified events (e.g. lung cancer). Prospective cohort studies (which track participants forward in time) are more reliable than retrospective cohort studies.

Case control study: a study design that examines a group of people who have experienced an event (usually an adverse event) and a group of people who have not experienced the same event, and looks at how exposure to suspect (usually noxious) agents differed between the two groups. This type of study design is most useful for trying to ascertain the cause of rare events, such as rare cancers.

Case Series: analysis of series of people with the disease (there is no comparison group in case series).

Find the Evidence







Thanks to ...

Joanne Rich at the Health Sciences Library at the University of Washington, for sharing her guide on evidence-based practice with us!