Definition
Measure of spread
A number that describes how close together or far apart data values are.
What it means
Centre tells you where a data set sits; spread tells you how tightly the values bunch together or how widely they scatter. Two data sets can have the same centre and still be very different because one has a small spread and the other has a large spread.
Remember it
Centre says where; spread says how far apart.
Example
The sets 4, 5, 6 and 1, 5, 9 both have median 5. Their ranges are 6 − 4 = 2 and 9 − 1 = 8, so the second set has the greater spread even though the centres match.
Watch for
A measure of spread does not tell where the data are centred. It must be interpreted alongside a measure of centre and the distribution’s shape.
Common mix-up
Range is one measure of spread, not another name for every way of measuring spread.
Exact meaning
A measure of spread is a numerical summary of the amount of variation or dispersion in a data set. Range is the main introductory example; later examples include interquartile range, mean absolute deviation, and standard deviation.