**Core Concept**
A reference range or reference interval is a set of values that define the normal range for a particular laboratory test or measurement. It serves as a basis for comparing individual patient results to determine if they fall within the expected range.

**Why the Correct Answer is Right**
In medical research and clinical practice, reference ranges are essential for interpreting laboratory results. They are established through studies that involve collecting data from a large and representative population. The mean and standard deviation of the population are then used to define the reference range. For example, the reference range for serum creatinine levels in adults is typically defined as 0.6 to 1.2 mg/dL. Any value outside this range may indicate kidney dysfunction or other underlying conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a method for establishing a reference range.
**Option B:** This option is incorrect because it refers to a study design, but does not relate to the concept of a reference range.
**Option C:** This option is incorrect because it is a type of statistical analysis, but not a method for establishing a reference range.

**Clinical Pearl / High-Yield Fact**
When interpreting laboratory results, it's essential to consider the reference range used by the laboratory. Different laboratories may use different reference ranges, even for the same test, due to variations in equipment, reagents, and population demographics. Always verify the reference range used by the laboratory when interpreting patient results.

**Correct Answer:** C. Control.