This is the fourth of six segments that show you how to improve the ozone prediction surface.

It is common practice to limit the data used by defining a circle (or ellipse) to enclose the points that are used to predict values at unmeasured locations.

Additionally, to avoid bias in a particular direction, the circle (or ellipse) can be divided into sectors from which an equal number of points are selected. By using the Searching Neighborhood dialog box, you can specify the number of points (a maximum of 200), the radius (or major/minor axis), and the number of sectors of the circle (or ellipse) to be used for prediction.

The points selected in the data view window indicate the weights that will be associated with each location in the prediction of unknown values. In this example, some locations (shown in red) have weights of more than 10 percent. The larger the weight, the more impact that location will have on the prediction of unknown values.

Click inside the graph view to select a prediction location (where the crosshairs meet). Note the change in the selection of data locations (together with their associated weights) that will be used for calculating the value at the prediction location.

For the purpose of this tutorial, type the following coordinates in the Identify text boxes: X = -2044968 and Y = 208630.37

Uncheck the Ellipse Default check box and type "90" in the Angle text box. Notice how the shape changes. However, to account for the directional influences, change the angle back to 339.6.

Check the Ellipse Default check box. The Geostatistical Analyst will use the default values (these were calculated on the Semivariogram/Covariance Modeling dialog box earlier). Click Next.