Try each of the classification types and see if any interesting spatial patterns appear. This is the default classification method in ArcView. This method is rather complex, but basically the Jenk s method minimizes the sum of the variance within each of the classes. Natural Breaks finds groupings and patterns inherent in your data. In the quantile classification method, each class contains the same number of features.
Quantile classes are perhaps the easiest to understand, but they can be misleading.
You can overcome this distortion by increasing the number of classes. Imagine the difference, for example, if five classes are used in the chart instead of three. This method classifies polygon features by finding breakpoints so that the total area of the polygons in each class is the approximately the same. Classes determined with the equal area method are typically very similar to Quantile classes when the sizes of all the features are roughly the same. Equal Area will differ from Quantile if the features are of vastly different areas.