MEASURES OF potential impact
The Number Needed to Treat
Risk difference can be used to compute “number needed to treat” or NNT - which represents the number of patients treated to prevent one outcome.
Example - study of patients with mild hypertension.
· Treatment of hypertension with b-antagonist/diuretic vs. placebo.
RD = 2.6% – 1.4% = 1.2% or .012
How many hypertensive patients need to be treated to prevent 1 stroke?
NNT = 1 / .012 = 83.333
Results: treat 83 hypertensive individuals for 10 years, 1 stroke prevented.
6-2 Measures of Potential Impact (continued)
Potential Impact – The Concept
v The etiologic fraction: measure of impact when exposure is a potential cause of disease.
v The prevented fraction: measure of impact when exposure prevents disease.
The etiologic fraction [population] answers the question: what proportion of new cases are attributable to the exposure?
The etiologic fraction [pop] of .35 tells us that 35% of all cases are due to continuing smoking.
In other words, if all patients had quit smoking after their heart attack, there would have been 35% fewer deaths deaths.
ID = 40 / 104,820 = 38.2 per 100,000 p-yrs
53% of all deaths among continuing smokers were due to continued smoking.
6-3 Measures of Potential Impact (continued)
(Note: RR should be 0.04 above)
55% of the total expected cases were prevented by immunization.
Using person-time data,
The prevented fraction of .96 among the exposed tells us that 96% of total expected cases among those immunized were prevented by immunization.