"Case in point" is a specific example used in discourse to illustrate a point or serve as an affirmation or a demonstration of a point. It is used in a variety of modes of discourse, from oratory and rhetoric to scientific studies and magazine articles. The term is used in speech and writing as a linking phrase between the idea and the example itself.
The phrase is an idiom related to "in point of fact." An idiom is a non-literal phrase of clause that derives a meaning from two or more normal words. "Case in point" contains and older idiom "in point" that is considered a fossil idiom. This is similar to a fossil word, as it only appears within another idiom. Other examples of fossil words include "ulterior" in "ulterior motives" and "kith" in "kith and kin."
“In point” comes from the Anglo-Norman idiom en point, which means "on the point," or more literally, "relevant." It probably comes from putting the point of a dagger, knife, or sword exactly where it needs to be. In 1659, Thomas Burton used the term in a description of Lord Fairfax’s political machinations.
Case comes from the Latin word casus, meaning a "chance event." It is used commonly by police to refer to an event they are investigating and is combined with history to mean medical records. "Case in point" probably came from a misunderstanding of Anglo-Norman. The original could have been "case en point" or "en point case," where a person was referring to a relevant example. It is likely that "en" was mispronounced as "in" and the idiom changed before being combined with "case."
Discourse usually places this phrase in the middle. The discourse will open with an introduction of the topic, and then it fleshes out the topic with a full description. The example is introduced to illustrate the topic, whether relevant to the problem or solution. Discourses tend to round off with analysis and a conclusion.
This example can be introduced in a number of ways. The simplest is the one-line introduction: "A is more effective than B; the C study is a case in point." Alternatively, the phrase will be introduced briefly, then expanded upon. In such a case, the "C study" will then provide testimony, details, and statistics to back up the point the orator or writer is trying to make.