In statistics, a graph or data set organized to show the frequency of occurrence of each possible outcome of a repeatable event observed many times. Simple examples are election returns and test scores listed by percentile. A frequency distribution can be graphed as a histogram or pie chart. For large data sets, the stepped graph of a histogram is often approximated by the smooth curve of a distribution function (called a density function when normalized so that the area under the curve is 1). The famed bell curve or normal distribution is the graph of one such function. Frequency distributions are particularly useful in summarizing large data sets and assigning probabilities.