calculations are as follows: population denominator (size of the population); proportion of the population with the disease; proportion of the population exposed to the risk factor or the incremental proportion of the diseased population eligible for the proposed intervention (the latter requires the actual or estimated proportion who are currently receiving the interventions 'subtracted' from best practice goal from guidelines or targets, adjusted for likely compliance with the intervention); baseline risk – the probability of the outcome of interest in this or similar populations; and relative risk of outcome given exposure to a risk factor or relative risk reduction associated with the intervention. NEPP The formula for calculating the NEPP is where N = population size, Pd = prevalence of the disease, Pe = proportion eligible for treatment, ru = 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, Pe, is , where Pt is the proportion currently treated, Pb is the proportion that would be treated if best practice were adopted, and Pc 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)] PIN − ER − t The formula for calculating the PIN − ER − t is where N is the number of people in the population; Ip 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 where Pe 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: References Biostatistics