Histograms are similar to bar charts apart from the consideration of areas. In a bar chart, all of the bars are the same width and the only thing that matters is the height of the bar. In a histogram, the area is the important thing.
Example: Draw a histogram for the following information.
Height (feet): (Number of pupils) Relative frequency:
0-2 0 0
2-4 1 1
4-5 4 8
5-6 8 16
6-8 2 2
(Ignore relative frequency for now). It is difficult to draw a bar chart for this information, because the class divisions for the height are not the same. If you do not understand, try drawing a bar chart.
When drawing a histogram, the y-axis is labelled 'relative frequency'. You must work out the relative frequency before you can draw a histogram. To do this, first work out the standard width of the groups. Some of the heights are grouped into 2s (0-2, 2-4, 6-8) and some into 1s (4-5, 5-6). Most are 2s, so we shall call the standard width 2. To make the areas match, we must double the values for frequency which have a class division of 1 (since 1 is half of 2). Therefore the figures in the 4-5 and the 5-6 columns must be doubled. If any of the class divisions were 4 (for example if there was a 8-12 group), these figures would be halved. This is because the area of this 'bar' will be twice the standard width of 2 unless we half the frequency.
This is the running total of the frequencies. On a graph, it can be represented by a cumulative frequency polygon, where straight lines join up the points, or a cumulative frequency curve.
Height (cm) Frequency: Cumulative frequency:
0 - 100 4 4
100-120 6 10 (4 + 6)
120-140 3 13 (4 + 6 + 3)
140-160 2 15 (4 + 6 + 3 + 2)
160-180 6 21 (4 + 6 + 3 + 2 + 6)
180-220 4 25 (4 + 6 + 3 + 2 + 6 + 4)
These data are used to draw a cumulative frequency polygon by plotting the cumulative frequencies against the upper class boundaries.