Statistical purposes include estimating a population parameter, describing a sample, or evaluating a hypothesis.
In this example, "5.6 days" is a statistic, namely the mean length of stay for our sample of 20 hotel guests.
[2] Whether the estimator is unbiased in this case depends upon the sample selection process; see the inspection paradox.
The height of the members of a sample of 100 such men are measured; the average of those 100 numbers is a statistic.
Important potential properties of statistics include completeness, consistency, sufficiency, unbiasedness, minimum mean square error, low variance, robustness, and computational convenience.