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

What do I need to know?

• 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

1. Drawing

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