In statistics, a parameter is a measured quantity of a statistical population that summarizes or describes an aspect of the population, such as a mean or a standard deviation. It is a value that tells you something about a population and is the opposite of a statistic, which tells you something about a small part of the population. Parameters are numbers that describe the properties of entire populations, while statistics are numbers that describe the properties of samples. For example, the average income for the United States is a population parameter, while the average income for a sample drawn from the U.S. is a sample statistic. Parameters are usually Greek letters or capital letters, while statistics are usually Roman letters. In statistical inference, parameters are sometimes taken to be unobservable, and in this case, the statisticians task is to estimate or infer what they can about the parameter based on a random sample of observations taken from the full population.