Attributable risk is a great tool for public health.

But as you know, it exclusively refers to the exposed group of people.

Sometimes we're interested in quantifying

the effect of an exposure on the entire population,

and not only on those exposed to it.

This is when population attributable risk comes in handy.

In this lecture, I will explain how you can calculate

population attributable risk and how it differs from attributable risk.

Population attributable risk is the excess risk of

disease in the total study population that is attributable to the exposure.

The total study population includes both exposed and unexposed individuals.

Once you know the numerical value of the attributable risk,

you only need to multiply it by the prevalence of the exposure in the population,

and you can easily calculate the population attributable risk.

In the classroom example we've seen before,

the attributable risk was 0.2 over a one hour period.

Exactly half of the students were exposed to the boring lecturer,

which means that the prevalence of the exposure in the population was 0.5.

Hence, the population attributable risk was 0.2 times

0.5 equals 0.1 or 10 percent over a one hour period.

The interpretation of this is that among the entire study population,

an excess risk of 10 percent over one hour could be attributed to the boring lecturer.

Similarly, the population attributable risk percent is the proportion of

disease in the population that could be prevented by eliminating the exposure.

To calculate it, you first need to

estimate the risk of sleeping in the classroom among the entire sample.

This is six sleeping students over 20 equals 0.3 over the study period.

You then divide the difference between

the risk among the population and the risk among the unexposed,

0.3 minus 0.2, by the risk among the population, which is 0.3.

This gives us a population attributable risk of 33 percent.

In lay terms, 33 percent of all the cases

of sleeping in the classroom could be attributed to the boring lecturer.

The value of the population attributable risk as a measure is obvious.

In contrast to the attributable risk which focuses on the exposed group,

it provides an insight into

the entire population which is frequently what we're interested in.

The population attributable risk percent depends on

both the prevalence of the exposure and the strength of the association.

Here, you can see

how different the population attributable risk percent can be depending on

how frequent is the exposure and how strong

the association between exposure and outcomes are.

For instance, a risk ratio of two can lead to

a population attributable risk percent of 0.5

or 33 percent depending on the prevalence of the exposure.

Attributable risk and population attributable risk provide valuable information about

the magnitude of the impact of an exposure which

cannot be captured by relative measures of association.

This is the kind of information you would need if you wanted to

prioritize public health interventions and maximize the benefit for the population.