Frequency Polygons
A frequency polygon is a useful graph with dots and lines that show the frequency (or counts) of quantitative data, sorted in class intervals. The height of the dots represents the frequencies of each interval.
Steps for Constructing a Frequency Polygon
- Decide on the number of intervals and size of intervals – like you would when constructing a histogram.
- Plot the dots at the midpoint of each interval.
- Connect the dots with lines from the lowest interval to highest interval.
Example:
Cumulative Frequency Polygon
A cumulative frequency polygon is just like a frequency polygon except that the intervals add up to 100% of the data from lowest to highest.
An interval contains the frequency of its own interval plus the sum of the frequencies of all the intervals that are smaller than it.
Example
Here is a cumulative frequency polygon for the average grade of students:
