The QQ plot is where you compare the distribution of the data to a standard normal distribution, providing another measure of the normality of the data. The closer the points are to the straight line in the graph, the closer the sample data follows a normal distribution.

To do so, click the Geostatistical Analyst drop-down arrow, point to Explore Data, then click Normal QQPlot. Click the Layer drop-down arrow and click ca_ozone_pts. Click the Attribute drop-down arrow and click OZONE.

A general QQ plot is a graph on which the quantiles from two distributions are plotted versus each other. For two identical distributions, the QQ plot will be a straight line. Therefore, it is possible to check the normality of the ozone data by plotting the quantiles of that data versus the quantiles of a standard normal distribution. From the normal QQ plot above, you can see that the plot is very close to being a straight line. The main departure from this line occurs at high values of ozone concentration (which were selected in the histogram plot, so they are selected here also).

If the data does not exhibit a normal distribution in either the histogram or normal QQ plot, it may be necessary to transform the data to make it conform to a normal distribution before using certain kriging interpolation techniques.

Close the Normal QQPlot dialog box.