In statistics there are many useful indices that we can use to describe data. Some common terms are median, mode and mean.

The weighted mean is similar to an arithmetic mean, where instead of each of the data points contributing equally to the final average, some data points contribute more than others. If all the weights are equal, then the weighted mean is the same as the arithmetic mean. For example, there are 4 15-year-old students, 13 16-year-old students, 8 17-year-old students, then the mean age will be ( 15 x 4 + 16 x 13 + 17 x 8 ) ÷ ( 4 + 13 + 8 ) = 16.16.