A statistic is a quantity that is calculated from a sample of data. It is used to give information about unknown values in the corresponding population. For example, the average of the data in a sample is used to give information about the overall average in the population from which that sample was drawn.
It is possible to draw more than one sample from the same population and the value of a statistic will, in general, vary from sample to sample. For example, the average value in a sample is a statistic. The average values in more than one sample, drawn from the same population, will not necessarily be equal.
Statistics are often assigned Roman letters, whereas the equivalent unknown values in the population (parameters ) are assigned Greek letters.