SPSS Statistical Graphing Tutorial: Frequency Polygon
1. Questions and data
A researcher wants to know whether the maximum oxygen carrying capacity (VO2max) in a data set obeys a normal distribution. Some data are shown in Figure 1. How should researchers view drawings?
Figure 1 Part of the data
2. Analysis of the problem
Researchers want to plot the distribution characteristics of the maximum oxygen-carrying capacity. In addition to simple histograms, frequency polygons can also be used. Frequency polygons are often used to display the distribution characteristics of continuous variables , especially to check whether continuous variables conform to normal distribution.
3. SPSS operation
3.1 Frequency polygon
Click Graphs→Chart Builder on the main interface and select Histogram in the Choose from box in the lower left corner. As shown in Figure 2.
Figure 2 Chart Builder
After selecting Histogram, 4 options are displayed on the right side of it. Drag the third graph to the upper preview pane (if you hover the mouse over the graph, Frequency polygon will be prompted). Figure 3.
Figure 3 Drag the Frequency polygon to the preview window
Drag the variable VO2max from the Variables: box to the "X-Axis?" box. As shown in Figure 4.
Figure 4 Drag the variable to the Frequency polygon
What needs to be noted here is that although the graph in the preview pane changes when a variable is added, it cannot plot accurately based on the data, so don't question yourself for making a mistake. In the end, the correct dot plot will be displayed based on the real data.
If the researcher wants to change the statistics displayed on the Y-axis, such as frequency or percentage, select "Area1" in the Edit Properties of box and select the corresponding statistics in the Statistics drop-down box.
3.2 Add normal curve
If you need to add a normal curve to help judge the normality of variables, you can check Display normal curve in the Edit Properties of. box, and click Apply to save the settings. Figure 5.
Figure 5 Adding a normal curve
3.3 Change the axis properties
To change the X-axis properties, click "X-Axis1 (Area1)" in the Edit Properties of dialog box. As shown in Figure 6.
Figure 6 Changing X-axis properties
You can then change the axis label (Axis Label box) or change the axis properties (Scale Range area). Uncheck the Minimum option in the Scale Range area, then the custom value (Custom) is highlighted and the default value is 0. In this example, the minimum value is set to 20, as shown in Figure 7.
Figure 7 Changing coordinate axis properties
This step is to display the reasonable value range of the X-axis variable, so the value displayed at Custom will change with different data, and the specific value needs to be set by the researcher. If the researcher is not sure about the exact values, please leave the values alone and check the resulting histogram. If necessary, reset this value to generate a suitable frequency polygon.
After all settings are completed, click OK.
4. Drawing results
Figure 8 is the final generated frequency polygon.
Figure 8 Frequency polygon