Sometimes the attribute information you have about map features is not organized the way you want—for instance, you have population data by county when you want it by state. By summarizing the data in a table, you can derive various summary statistics—including the count, average, minimum, and maximum values—and get exactly the information you want. ArcMap creates a new table containing the summary statistics. You can then join this table to the attribute table of a layer so you can symbolize, label, or query the layer's features based on their values for the summary statistics.
Summarizing with null values
- Null values are excluded from all statistical calculations. For example, the average of 10, 5, and NULL is 7.5 - (10 + 5) / 2. The count returns the number of values included in the statistical calculation, which in this case is 2.
- The count field (named with a cnt_ prefix) will find the number of values included in statistical calculations. This counts each value except null values. To determine the number of null values in a field, use the count statistic on the field in question and a count statistic on a different field that does not contain nulls (for example, the OID if present), then subtract the two values.
- Right-click the field heading of the field you want to summarize and click Summarize.
- Check the box next to the summary statistics you want to include in the output table.
- Type the name and location of the output table you want to create or click the browse button and navigate to a workspace.
- Click OK.
A new layer is added to the map.
- Click Yes when prompted to add the new table to your map.