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.