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Center for Health Sciences
Medical Physiology - Evidence-Based Medicine
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What is EBM?

Patient Care Model
Life-long Learning
Why is EBM Important?
Available Evidence?
EBM Issues

The Well-built Question

The EBM Process
Anatomy of a Question

Finding Evidence

Selecting a Resource
Searching the Resource
Reviewing Search Results
Returning to the Patient

Evaluating Evidence

Evaluating the Validity
Validity Questions


Knowledge Test

Multiple Sclerosis
Case #2
Case #3
Case #4




Evaluating the Validity of a Prognosis Study

1. Was there a representative and well-defined sample of patients at a similar point in the course of the disease?

Patients should be included in the study at a uniformly early point in the disease. The ideal time is when it first manifests itself clinically. This is referred to as an "inception cohort."

2. Was follow-up sufficiently long and complete?

Patients should be followed until they fully recover or one of the disease outcomes occurs. The follow-up should be long enough to develop a valid picture of the extent of the outcome of interest.

Follow-up should include at least 80% of participants until the occurrence of a major study end point or to the end of the study.

3. Was objective and unbiased outcome criteria used?

Some outcomes are clearly defined, such as death or full recovery. In between, a wide range of outcomes can exist that may be less clearly defined. Investigators should establish specific criteria that define each possible outcome of the disease and use these same criteria during patient follow-up. These should be listed in a table within the study text.

Investigators making judgements about the clinical outcomes may have to be "blinded" to the patient characteristics and prognostic factors in order to eliminate possible bias in their observations.

4. Was there adjustment for important prognostic factors?

In comparing the prognosis of the 2 study groups, researchers should consider whether or not the patients clinical characteristics are similar. It may be that adjustments have to made based on age or gender to get a true picture of the clinical outcome.

Key issues for Prognosis Studies:

  • well-defined sample 

  • follow up 

  • objective and unbias outcome criteria

  • adjustment for important prognostic factors


What are the Results: issues of prognosis

Prognosis of a disease refers to its possible outcomes and the likelihood that each one will occur.

Prognostic Results are the numbers of events that occur over time, expressed in:

  • absolute terms: e.g. 5 year survival rate  

  • relative terms: e.g. risk from prognostic factor  

  • survival curves: cumulative events over time

Prognostic Factors are patient characteristics that can predict the patient's eventual outcome:

  • demographic: e.g. sex, age, race 

  • disease-specific: e.g. tumor stage  

  • comorbidity: other co-existing conditions

Articles reporting Prognostic Factors often use two independent patient samples:

  • derivation sets asks - "what factors might predict patient outcomes?"

  • validation sets ask - "do these prognostic factors predict patient outcomes accurately?"

Source: Laupacis A ; Wells G ; Richardson WS ; Tugwell P. Users' guides to the medical literature. V. How to use an article about prognosis. Evidence-Based Medicine Working Group. JAMA 1994 Jul 20; 272(3):234-7.

Note: For criteria for other types of studies, see the following
supplements:  Therapy | Diagnosis | Etiology/ Harm

We now go to the next section, Knowledge Test: Case #1