In a population of 5000 people, 500 are already myopic on January 1, 2011, number of new myopic cases till December 31, 2011 is 9. Calculate incidence:

## **Core Concept**
The core concept being tested here is the calculation of **incidence rate** in epidemiology. Incidence rate refers to the number of new cases that develop in a specified time period among a population at risk. It is usually expressed as a rate, such as the number of new cases per 1,000 or 100,000 population per year.

## **Why the Correct Answer is Right**
The correct formula for incidence rate is: (Number of new cases during a specified time period / Population at risk at the beginning of the time period) * 1000 (or any other standard unit). Here, the population at risk at the beginning of the year (January 1, 2011) is 5000 - 500 (already myopic) = 4500 people. The number of new myopic cases till December 31, 2011, is given as 9. So, the incidence rate = (9 / 4500) * 1000.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option does not match our calculated formula and seems to ignore the population at risk.
- **Option B:** This option incorrectly uses the total population (5000) instead of the population at risk (4500) for calculating the incidence.
- **Option D:** This option seems to miscalculate or misrepresent the incidence rate formula.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **prevalence** includes both old and new cases, whereas **incidence** only includes new cases. The population at risk for developing a condition does not include those already affected by the condition.

## **Correct Answer:** .