320 Statistical Methods
Plotting Regression Data
Before you calculate any regression statistics, you should always plot your
data. A scatter plot can quickly point out obvious problems in assuming
that a linear model fi ts your data (perhaps the scatter plot will show that the
data values do not fall along a straight line). Scatter plots in Excel also allow
you to superimpose the regression line on the plot along with the regression
equation. From this information, you can get a pretty good idea whether a
straight line fi ts your data or not.To create a scatter plot of the mortality data:1 Click Single Variable Charts from the StatPlus menu and then click
Fast Scatter plot.
2 Click the x axis button, select the Temperature range name and click OK.
Click the y axis button, select the Mortality range name and click OK.
3 Click the Chart Options button, enter Mortality vs. Temp for the
Chart title, Temperature for the x axis title, and Mortality for the y
axis title. Click OK.
4 Click the Output button and save the chart to a new chart sheet
named Mortality Scatter plot. Click OK.
5 Click the OK button to generate the scatter plot.
6 In the scatter plot that’s created, resize the horizontal scale so that the
lower boundary is 30 , and then resize the vertical scale so that the lower
boundary is 50. Figure 8-4 shows the fi nal version of the scatter plot.Figure 8-4
Scatter plot
of the
mortality
index versus
mean annual
temperature