1. It is a sum total of new and old cases.
2. It refers to a specified period of time.
3. It refers to a defined population.
4. It must include the unit of time in final expression.
Select the correct answer using the code given below.
Incidence Rate: Correct Characteristics
Incidence rate measures the frequency of new cases of a disease occurring in a defined population during a specified period of time. The correct answer is 2, 3 and 4.
Evaluation of Each Statement
- Statement 1: "It is a sum total of new and old cases." — Incorrect. Incidence strictly counts only new cases arising during the observation period. Old (pre-existing) cases are excluded from incidence; a measure that includes both new and old cases is prevalence, not incidence. This is a common point of confusion, but it is a defining feature that distinguishes incidence from prevalence.
- Statement 2: "It refers to a specified period of time." — Correct. Incidence rate is always calculated over a defined time interval (e.g., per day, month, or year), since it reflects the rate at which new cases develop.
- Statement 3: "It refers to a defined population." — Correct. Incidence is expressed as new cases divided by the population at risk during that period, so it is inherently population-specific (e.g., per 1,000 population).
- Statement 4: "It must include the unit of time in final expression." — Correct. Because incidence reflects the rate of occurrence of new events, its final expression must state the time dimension (e.g., "cases per 1,000 population per year"), which is what makes it a true "rate."
Why the Other Options Are Wrong
- 1, 3 and 4: Incorrectly includes statement 1 (which wrongly adds old cases to the definition of incidence) and omits statement 2 (time period), which is essential to defining incidence.
- 1, 2 and 3: Incorrectly includes statement 1 and omits statement 4 (the requirement to state the time unit), which is a core feature of expressing any rate.
- 1, 2 and 4: Incorrectly includes statement 1 and omits statement 3 (defined population), which is necessary since incidence rate is always calculated relative to a specific population at risk.
Thus, the statements that correctly describe incidence rate are 2 (specified time period), 3 (defined population), and 4 (unit of time in the final expression) — corresponding to option 2, 3 and 4.
Important Questions from General Epidemiology
Which of the following statements are correct for incidence rate?
1. It is useful for taking action to control a disease.
2. Rising incidence rates may indicate ineffectiveness of the current control programmes.
3. Fluctuation in incidence rate may indicate a change in aetiology of disease.
Select the answer using the code given below.
Consider the following definition :
"A high level of infection beginning early in life and affecting most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than the children."
Which one of the following terms best fits this definition?
- In which one of the following study designs, the unit of study involves populations rather than individuals ?
- Which one of the following best explains the relationship among Prevalence (P), Incidence (I) and Duration (D) of a disease given the assumption that the population is stable?
- Which one of the following is a famous large prospective study that helped establish risk factors for coronary heart disease?
