Measures of effect and impact

Disease status	Exposed	Unexposed	Total
Diseased	a1	a0	m1
Non-diseased	b1	b0	m0
Total	n1	n0	n

Risk difference (RD)

Risk difference or Attributable risk is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.

Attributable fraction (AF)

The attributable fraction is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.

Population attributable risk (PAR)

The population attributable risk is analogous to the risk difference in that it gives a difference in the risk between two groups; however PAR is an estimate of the amount of disease in the total population attributed to the risk factor. It is calculated as the overall risk in the population minus the risk in the unexposed group or by multiplying the risk difference by risk of disease in the population. PAR can only be estimated from studies where the groups selected are representative of the target population.