This figure is from the Fisher paper.

It shows diarrhea episodes per child per year in selected low income countries.

The range is about 2 to 4 episodes per child per year.

The second parameter is the effectiveness of

the intervention at reducing the incidence of

WASH-related diseases in the population if a

specified number of individuals use the intervention.

You can think of the effectiveness of the

intervention as the percentage reduction in baseline incidence.

So, if an intervention is 90% effective, you have 10% of the number of cases left.

We'll assume, for purposes of illustration, that 100% of

the people in the target population use the intervention.

The instance after the intervention, labeled here Inc subscript after, is 1

minus Eff times the baseline incidence,

that is the instance before the intervention.

The change in the number of cases due to the intervention is the number

of cases in the target population before

minus the number of cases after the intervention.

To calculate the change in the number of cases, you multiply the number of

people in the target population times the

effectiveness of the intervention, times the baseline incidence.