Population impact measure

The formula for calculating the NEPP is

${\displaystyle {\text{NEPP}}=N\times P_{d}\times P_{e}\times r_{u}\times {\text{RRR}}}$

where

• N = population size,
• P_d = prevalence of the disease,
• P_e = proportion eligible for treatment,
• r_u = risk of the event of interest in the untreated group or baseline risk over appropriate time period (this can be multiplied by life expectancy to produce life-years),
• RRR = relative risk reduction associated with treatment.

In order to reflect the incremental effect of changing from current to 'best' practice, and to adjust for levels of compliance, the proportion eligible for treatment, P_e, is ${\textstyle (P_{b}-P_{t})P_{c}}$, where P_t is the proportion currently treated, P_b is the proportion that would be treated if best practice were adopted, and P_c is the proportion of the population who are compliant with the intervention.

[Note: number needed to treat (NNT): 1/(baseline risk x relative risk reduction)]

The formula for calculating the PIN − ER − t is

${\displaystyle {\text{PIN}}-{\text{ER}}-t=NI_{p}\cdot {\text{PAR}}}$

where

• N is the number of people in the population;
• I_p is the baseline risk of the outcome of interest in the population as a whole;
• t is the amount of time over which the outcome is measured.

The PAR/F, population attributable risk (or fraction), is calculated for two or multiple strata. The basic formula to compute the PAR for dichotomous variables is

${\displaystyle {\text{PAR}}=P_{e}{\frac {{\text{RR}}-1}{1+P_{e}({\text{RR}}-1)}}}$

where

• P_e is the prevalence of the population within each income stratum as the exposure, and
• RR is the prevalence of risk factors in each stratum relative to the highest income fifth.

This is modified where there are multiple strata to:

${\displaystyle {\text{PAR}}={\frac {P_{e1}({\text{RR}}_{1}-1)+P_{e2}({\text{RR}}_{2}-1)+P_{e3}({\text{RR}}_{3}-1)+\cdots }{1+P_{e1}({\text{RR}}_{1}-1)+P_{e2}({\text{RR}}_{2}-1)+P_{e3}({\text{RR}}_{3}-1)+\cdots }}}$