Variance

The average of the squared deviation (or difference) of a variable from the mean; this identifies the extent of the dispersion or spread of the values. The variance is helpful for making inferences from data, although standard deviation (which is the square root of the variance) is often the preferred method of determining variability in data.

Two methods of variance are used, either population variance (where all data of a group are included – this would only be applicable for small surveys) or sample variance (where a sample of a group is being used and is probably the most common approach).