LESSON 6

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.

Etiologic Fraction [Population]

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.

Etiologic Fraction for Person-Time Studies

ID = 40 / 104,820 = 38.2 per 100,000 p-yrs

Etiologic Fraction among the Exposed

53% of all deaths among continuing smokers were due to continued smoking.

6-3 Measures of Potential Impact (continued)

Prevented Fraction [Population]

(Note: RR should be 0.04 above)

55% of the total expected cases were prevented by immunization.

Using person-time data,

Prevented Fraction among the Exposed

The prevented fraction of .96 among the exposed tells us that 96% of total expected cases among those immunized were prevented by immunization.