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IMPORTANT CONCEPTS

PROGNOSIS



Researchers trace the progression of a disease in a variety of ways.
They will often -but not always - talk about a group of people possessing a prognostic factor and a group that does not and separate those groups into those people who present the outcome and those who do not.  For example, if we wanted to study whether diabetes is a prognostic factor for renal failure, we would set up the following table.


Patients with Renal Failure
Patients without Renal Failure
Patients with Diabetes
150
850
Patients without Diabetes
30 970





Risk
The Risk of an outcome is the number of times the outcome occurred as a percentage of all possible occurrences. In the case above, the risk of a patient with diabetes developing renal failure is 150/(150+850) or 15%.   In other words, a patient with diabetes has a 15% chance of developing renal failure.  Risk is often reported in descriptive articles where there is no control group.


Relative Risk
The Relative Risk of an outcome occurring is the risk of the outcome occurring in the group presenting the prognostic factor as a percentage of the risk of the outcome occurring in the group that did not present the prognostic factor.  In the example above, the risk of a patient with diabetes developing renal failure is 15% (150/1000).  The risk of a patient without diabetes developing renal failure is 3% (30/1000).  The Relative Risk is 15%/3% or 5.  In other words, patients with diabetes are 5 times more likely to have renal failure than patients without.  Relative Risk is also reported as Relative Risk Ratio or Risk Ratio.


Odds
The odds of an outcome occurring is the number of times the outcome occurred in patients with the prognostic factor as a percentage of the times the outcome occurred in patients who did not present with the prognostic factor.  In the example above, the odds of a patient with diabetes developing renal failure would be 150/30 or 5.  In other words the odds are 5:1 that a patient with diabetes will develop renal failure

Odds Ratio
The Odds Ratio of an outcome occurring is the odds that the outcome occurs in the group presenting the prognostic factor as a percentage of the odds of the outcome not occurring in the group that presents the prognostic factor.  In the example above the odds of a patient with diabetes developing renal failure was calculated as 5.  The odds of a patient with diabetes not developing renal failure is .87  (850/970).  The Odds Ratio would be 5/.87 or 5.7. In other words, a patient with diabetes, all other things being equal, is 5.7 times more likely to develop renal failure than not. 

Relative Risk vs. Odds Ratio
Whether researchers report Relative Risk or Odds Ratio depends on whether they do a cohort study or a case control study.   Because of the way subjects are chosen in a case control study, the percentages may be skewed.  The Odds Ratio overcomes this problem by having both factors in the denominator.


Survival Curves
A survival curve is a graphic representation of risk over time.  Rather than presenting the risk at one particular endpoint, risk of death is computed at given intervals and presented graphically.

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