Frequency-Polygons Wjec-Eduqas Foundation

Frequency polygons

What are the key features of a frequency polygon?

• 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 2 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

How do I draw a frequency polygon?

• This is easiest shown by an example

  • e.g. The lengths of 59 songs, in seconds, are recorded in the table below

• Frequencies are plotted at the midpoints of the class intervals

  • so in this case we would plot the points (135, 4), (165, 10), (195, 24), (225, 18) and (255, 3)

  • Join these up with straight lines (but do not join the last to the first!)

How do I use and interpret a frequency polygon?

• Think about what you could 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 (an estimate of) the modal song length

    • The diagram (rather than the table) shows (an estimate of) the range of song lengths is 255 – 135 = 120 seconds

    • If 2 frequency polygons are drawn on the same graph comparisons between the 2 sets of data can be made