What is a statistic?

• A. A descriptive value based on a sample from the population, which is used to estimate population parameter.
• B. A descriptive value based on the entire population.
• C. A probability value associated with a model.
• D. A random variable.

Answer: (A)

A statistic is a descriptive value calculated from a sample that is used to estimate a population parameter. Think of measuring a subset of individuals and summarizing that subset with a number—like the sample mean or sample proportion. That value serves as an estimate of the true, but often unknown, population value (the parameter).

For example, the average height you measure in a sample of people from a city is a statistic that estimates the city’s overall average height. The distinction is that the true population mean is fixed but usually unknown, while the statistic you compute from your sample can vary from one sample to another.

Why the other ideas don’t fit the best definition: a descriptive value based on the entire population would describe the parameter itself, not a statistic. A probability value tied to a model is a p-value or similar concept, not the general descriptor of a sample-based statistic. And while a statistic can be treated as a random variable (since it changes with different samples), the core idea is that it is a sample-derived descriptor used to estimate a population parameter.