Incidence of diarrhoea in a community can be calculated by –
**Core Concept**
The incidence of diarrhoea in a community is a measure of the number of new cases of diarrhoea that occur within a specified time period. It is an important epidemiological parameter used to assess the burden of diarrheal diseases in a population.
**Why the Correct Answer is Right**
The correct method to calculate the incidence of diarrhoea is by using the formula: Incidence = Number of new cases / Person-time at risk. This means that the numerator represents the number of new cases of diarrhoea that occur within a specified time period, and the denominator represents the total time at risk for the population being studied. This formula takes into account the duration of follow-up for each individual, making it a more accurate measure of the true incidence of diarrhoea.
**Why Each Wrong Option is Incorrect**
**Option A:** Incidence is not calculated by dividing the number of cases by the population size, as this would give a prevalence rather than an incidence.
**Option B:** Incidence is not calculated by multiplying the number of cases by the duration of illness, as this would give a measure of the total number of cases rather than the new cases occurring within a specified time period.
**Option C:** Incidence is not calculated by dividing the number of cases by the number of deaths, as this would give a measure of the case-fatality rate rather than the incidence of diarrhoea.
**Clinical Pearl / High-Yield Fact**
To remember the formula for calculating incidence, use the mnemonic "Numerator New Cases / Denominator Duration" (NN/DD).
**Correct Answer:** D. Incidence = Number of new cases / Person-time at risk