# 9.4.1 Frequency Polygons

#### 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

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

#### Exam Tip

Jot down the midpoints next to the frequencies so you are not trying to work them out in your head while also concentrating on actually plotting the points.

### Worked Example

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