Descriptive Statistics

Averages are examples of some descriptive statistics and can be used to supply a quick picture of a set of numbers.
inter-relationships between pairs of variables

Measures of Central Tendancy

The average is one kind of descriptive statistic and can be thought of as a "measure of central tendency"

Example small spread with average 15
Example large spread with average 15

Measures of Dispersion

Therefore we also need to be able to give a statistic for the variation or the size of the spread.
This can be thought of as a "measure of spread" or "measure of dispersion"

We need statistics to be able to describe adequately the large number of people or whatever we are studying both in terms of typical patterns and the variation which we might expect

Distributions are used to describe a set of numbers
Distributions or groups of numbers are often represented as diagrams.