# Epidemiology – Attributable Risk (including AR% PAR + PAR%)

These are really important measures for public health as they indicate the magnitude of risk in absolute terms.

Attributable Risk (AR)

• AR is the portion of disease incidence *in the exposed* that is due to the exposure.
• Therefore = the incidence of a disease *in the exposed* that would be eliminated if the exposure were eliminated
• Calculation of AR = risk(incidence) in exposed – risk(incidence) in non-exposed which provides the risk difference

Attributable Risk % (proportion or fraction)

• AR is sometimes expressed as a proportion (%) of the disease incidence in the exposed – this is the proportion of disease incidence *in the exposed* that is due to the exposure.
• Therefore it is the proportion of the disease incidence *in the exposed* that would be eliminated if exposure were eliminated
• Calculation of AR% = AR / risk(incidence) in exposed x 100%
• …When data on disease incidence is not available we can use the RR…
• Calculation of AR% = (RR-1) / RR x 100%

Population Attributable Risk (PAR)

• This is a similar measure to AR except it is concerned not with the excess rate of disease *in the exposed* but the excess rate of disease *in the population* (compared with the rate of disease in the exposed group)
• PAR is the proportion of the disease incidence *in the population* (i.e. exposed and non-exposed) that is due to the exposure
• Therefore it is the disease incidence *in the population* that would be eliminated if the exposure were eliminated
• Calculation of PAR = risk(incidence) in population – risk(incidence) in non-exposed

Population Attributable Risk % (porportion or fraction)

• PAR% is the proportion of disease incidence *in the population* (i.e. exposed and non-exposed) that is due to the exposure
• Therefore it is the % of disease incidence *in the population* that would be eliminated if the exposure were eliminated
• Calculation of PAR% = PAR / risk(incidence) in population
• …When data on disease incidence is not available we can use the RR…
• Calculation of PAR% = prevalence in exposed population x (RR-1) / [1+ prevalence in exposed population (RR-1)]

PAR% = important indice in prioritising population interventions

• However, it assumes that all the association between disease and exposure is causal… and PAR varies according to how common an exposure to the risk factor is in the population
• AR + PAR are hypothetical constructs… there is no temporal depth
• Important for the counterfactual to be defined in order to explain their meaning (e.g. if smoking = lung cancer, no smoking = no lung cancer)
• Based on logic of risk subtraction (rather than risk explanation)