**Core Concept**: In social medicine, a deviation is a specific way of categorizing and reporting epidemiological data. The most common deviations used in social medicine include the "By-person", "By-event", and "By-period" deviations.

**Why the Correct Answer is Right:**
The correct answer, "By-event", involves counting the number of events for each individual over a specified period. For example, if we are studying the incidence of hypertension, we would count each person who develops hypertension as a new event, and record the time period when the event occurred. This method helps in understanding the occurrence of different diseases or health conditions in a population.

**Why Each Wrong Option is Incorrect:**
A) "By-person" deviation focuses on counting all individuals within a defined population over a specific time period. This method is useful for studying prevalence but not incidence.

B) "By-event" deviation, as explained above, is for incidence studies.

C) "By-period" deviation is the least common method used as it focuses on counting events or diseases that occur within a defined population during a specific time period. This method is useful for studying prevalence but not incidence.

**Clinical Pearl / High-Yield Fact:**
In epidemiological studies, choosing the right deviation depends on the research question (incidence or prevalence) and the type of data you want to analyze. Understanding these deviations is crucial for interpreting epidemiological findings accurately.

**Correct Answer:** By-event (Option B).