Measures of Spread

The 'spread' of a set of data is a description of how the values of that data are distributed/dispersed.

Range
Range is the simplest measure of spread, it is simply the difference between the largest(maximum) and smallest(minimum) values in a set of data.
For example: the range for the data
1,2,2,3,3,3,4,4,5
is 5 - 1 = 4.
The range tells us little about the distribution of the data and is greatly influenced by outliers.

Quartiles
The quartiles are 3 numbers that divide a distribution into quarters.
The 1st quartile Q₁ is the median of the first half of the distribution.
The 2nd quartile Q₂ is the median of the whole distribution.
The 3rd quartile Q₃ is the median of the second half of the distribution.

Five-Number Summary
A five-number summary is a numerical summary of a distribution, it gives the minimum, Q₁, median, Q₃, and maximum - in that order.

For example: the five-number summary for
1,2,2,3,3,3,4,4,5,6,7
is
1 2.5 3 4.5 7

Boxplot
Boxplots are graphical representations of five-number summaries.
To make a boxplot:
1. Draw a box between the the first and third quartiles.
2. Draw a line across the box at the median.
3. Draw lines extending out from the box at either end to the maximum and minimum.
4. Outliers can be marked separately on the diagram by a cross, box, line, etc.

Box plot of five-number summary in example above.