∂ is a parameter value, usually unknown (and which has to be estimated), is used to represent a certain population characteristic. For example, if you are studying the population of the United States the mean age of all inhabitants of the United States is a parameter.

            Within a population, a parameter is a fixed value, which dies not vary. Each sample drawn from the population has its own value of any statistic that is used to estimate this parameter. For example, the mean of the data in a sample is used to give information about the overall mean in the population from which that sample was drawn.

            Parameters are often assigned Greek letters (e.g. µ (mu) for the population mean and lowercase sigma for the population standard deviation), whereas statistics are assigned Roman letters (e.g. where ª (bar x) is the sample mean and s is the sample variance).