- Frequency Polygons
A frequency polygon is another type of frequency distribution graph. In a frequency polygon, the number of observations is marked with a single point at the midpoint of an interval. A straight line then connects each set of points. Frequency polygons make it easy to compare two or more distributions on the same set of axes. (Hennekens, 1987, p. 218)
Let’s look at an example of a frequency polygon.
Source: CDC 1998, p.241
Notice the dotted outline of a histogram for the same data. A frequency polygon smoothes out the abrupt changes that may appear in a histogram, and is useful for demonstrating continuity in the variables being studied. In this example, the number of reported cases of influenza-like illness peaked during week 4 after the onset of illness.
Like a histogram, frequency polygons are used to display the entire frequency distribution (counts) of a continuous variable. They must be closed at both ends because the area under the curve represents all of the data. By contrast, an arithmetic-scale line graph represents a series of observed data points (counts or rates), usually over time – it simply plots data points.