Frequency Polygon

Relative frequencies of class intervals also can be shown in a frequency polygon. In this chart, the frequency of each class is indicated by points or dots drawn at the midpoints of each class interval. Those points are then connected by straight lines.

Comparing the frequency polygon (shown in Figure 1) to the frequency histogram (refer to Figure 1 in "Frequency Histogram"), you see that the major difference is that points replace the bars.

Figure 1.Frequency polygon display of items sold at a garage sale.

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Whether to use bar charts or histograms depends on the data. For example, you may have categorical (or qualitative) data—numerical information about categories that vary significantly in kind. Gender (male or female), types of automobile owned (sedan, sports car, pickup truck, van, and so forth), and religious affiliations (Christian, Jewish, Muslim, and so forth) are all qualitative data. On the other hand, quantitative data can be measured in amounts: age in years, annual salaries, inches of rainfall. Typically, qualitative data are better displayed in bar charts; quantitative data, in histograms.