**Core Concept**
Incidence rate is a measure of the number of new cases of a disease or condition that occur within a population over a specified period of time. It is an important epidemiological parameter used to assess the risk of developing a disease.

**Why the Correct Answer is Right**
The incidence rate is typically measured using a cohort study, which involves following a group of individuals who share a common characteristic (e.g., age, occupation, or exposure to a risk factor) over time to determine the number of new cases that occur. This design allows researchers to estimate the incidence rate by dividing the number of new cases by the total person-time at risk. In a cohort study, the population is followed from the beginning of the study period, making it an ideal design for measuring incidence rates.

**Why Each Wrong Option is Incorrect**
* **Option A:** Case control studies are used to measure the association between a disease and a potential risk factor, but they are not designed to measure incidence rates. Case control studies involve selecting individuals with the disease (cases) and comparing them to individuals without the disease (controls) to identify potential risk factors.
* **Option C:** Cross-sectional studies are used to measure the prevalence of a disease at a single point in time, but they are not designed to measure incidence rates. Prevalence is the total number of cases (new and existing) divided by the total population.
* **Option D:** Cross-over studies involve each participant serving as their own control, receiving different treatments or interventions in different study periods. While cross-over studies are useful for comparing the effects of different treatments, they are not designed to measure incidence rates.

**Clinical Pearl / High-Yield Fact**
Remember that the key difference between incidence and prevalence is that incidence measures the rate of new cases, while prevalence measures the total number of cases at a given time.

**✓ Correct Answer: B. Coho study. Cohort study is used to measure incidence rate.**