1. To accompany Hawkes lesson 2.2b(iii) Original content by D.R.S.
Frequency Polygon Input: A frequency distribution Output: A pretty picture
To accompany Hawkes lesson 2.2b(iii)
Original content by D.R.S.
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2. Remember Polygons from Geometry?
It’s a shape
It’s a closed figure
All the sides are straight lines meeting at their endpoints
The sides never cross each other
Triangle, Quadrilateral, Pentagon, Hexagon, Heptagon, Octagon, Nonagon, Decagon, etc…
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3. Step 1 – A Frequency Distribution
Class Limits
Frequency 9 26 23 14 7 11 10 2 1
These are the runners’ times from the 5K race again.
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4. Need Class Midpoints (Low + High) / 2
Class Limits
Class Midpoint
Frequency 9 26 23 14 7 11 10 2 1
(Low + High) / 2
Frequency Polygons plot the Class Midpoints on the horizontal axis
Unlike Histogram, which used class boundaries.
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5. Add Phony Classes at the Endpoints
Class Mid.
Frequency 9 26 23 14 7 11 10 2 1
Lowest midpoint minus one class width, – 5
Highest midpoint plus one class width,
Each with frequency 0.
These will “anchor” the polygon to the horizontal axis.
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6. The Frequency Polygon
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7. The Frequency Polygon
Good charts have a title telling the reader what he is looking at
Descriptive labels
on the axes, too
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8. Horizontal Axis close-up
Labels at x-values are class midpoints, not boundaries.
The phony classes we added at the beginning and end of the frequency distribution serve to “anchor” the polygon to the axis, so it is a closed geometric shape.
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9. Frequency Polygon in Excel
See handout with details.
Excel’s Line Graph is used.
Extra work is needed to set up phony classes.
Extra work is needed to get the labeling right.
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