What are frequency polygons?
Frequency polygons are a very simple way of showing frequencies for continuous, grouped data and give a quick guide to how frequencies change from one class to the next.
Apart from plotting and joining up points with straight lines there are two rules for frequency polygons:
- Plot points at the MIDPOINT of class intervals
- Unless one of the frequencies is 0 do not join the frequency polygon to the x-axis
and do not join the first point to the last one
The result is not actually a polygon but more of an open one that ‘floats’ in mid-air!
You may be asked to draw a frequency polygon and/or use it to make comments and compare data.
eg The lengths of 60 songs, in seconds, are recorded in the table below.
Draw a frequency polygon for these data.
2. Using and interpreting
What can you say about the data above, particularly by looking at the diagram only?
The two things to look for are averages and spread.
The modal class is 180≤t<210.
It would be acceptable to say that 195 seconds is the modal song length.
The diagram (rather than the table) shows the range of song lengths is 255 – 135 = 120 seconds.
If two frequency polygons are drawn on the same graph comparisons between the two sets of data can be made.