This frequency table is shown in Table 2–1. The number of observations in each class is called the class frequency. So the class frequency for domestic vehicles sold is 50. FREQUENCY TABLE A grouping of qualitative data into mutually exclusive classes showing the number of observations in each class. TABLE 2–1 Frequency Table for Vehicles Sold at Whitner Autoplex Last Month Car Type Number of Cars Domestic 50 Foreign 30 Relative Class Frequencies You can convert class frequencies to relative class frequencies to show the frac- tion of the total number of observations in each class. So, a relative frequency cap- tures the relationship between a class total and the total number of observations. In the vehicle sales example, we may want to know the percent of total cars sold that were domestic or foreign. To convert a frequency distribution to a relative frequency distribution, each of the class frequencies is divided by the total number of observations. For example, 0.625, found by 50 divided by 80, is the fraction of domestic vehicles sold last month. The relative frequency distribution is shown in Table 2–2.
Graphic Presentation of Qualitative Data The most common device to present a qualitative variable in graphic form is a bar chart. In most cases the horizontal axis shows the variable of interest and the verti- cal axis the amount, number, or fraction of each of the possible outcomes. A distin- guishing characteristic of a bar chart is the distance or gap between the bars. That is, because the variable of interest is qualitative, the bars are not adjacent to each other. Thus, a bar chart graphically describes a frequency table using a series of uni- formly wide rectangles, where the height of each rectangle is the class frequency. Describing Data: Frequency Tables, Frequency Distributions, and Graphic Presentation 23 BAR CHART A graph in which the classes are reported on the horizontal axis and the class frequencies on the vertical axis. The class frequencies are proportional to the heights of the bars. Foreign C ar Type (Variable of intere s t) Number S old ( C la ss frequency) Dome s tic 0 10 20 30 40 50 6 0 CHART 2–1 Vehicle Sold by Type Last Month at Whitner Autoplex TABLE 2–2 Relative Frequency Table of Vehicles Sold by Type at Whitner Autoplex Last Month Vehicle Type Number Sold Relative Frequency Domestic 50 0.625 Foreign 30 0.375 Total 80 1.000 We use the Whitner Autoplex data as an example (Chart 2–1). The variable of inter- est is the vehicle type and the number of each type sold is the class frequency. We scale the vehicle type (domestic or foreign) on the horizontal axis and the number of each item on the vertical axis. The height of the bars, or rectangles, corresponds to the number of vehicles of each type sold. So for the number of foreign vehicles sold the height of the bar is 30. The order of foreign or domestic on the X- axis does not matter because the values of car type are qualitative.
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