Massachusetts Institute of Technology
Department of Urban Studies and Planning

11.220 Quantitative Reasoning and Statistical Methods for Planning

ArcGIS

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ArcGIS > Statistics

1. Right-click on the block group layer and choose "Open Attribute Table."

2. The outcome is the following.

3. Locate the " tbl_rec4.pct_own" column. Right-click on the field name and select "Statistics..."

4. Look at the statistics about home ownership in the selected 80 blockgroups.

5. Switch the selection by clicking Options->Switch Selection.

7. Get the statistics about the other set of block groups.

8. You can do a t-test to compare the two means of percent home ownership between the within-buffer and the out-of-buffer blockgroups with these statistics. The formula can be found here.

Add a field to an ArcMap attribute table.

If you want to permanently store an indicator of "inside" or "outside" the buffer for each of your census block group polygons you should add a field to the attribute table. Here's how.

1. With the attribute table open, click on the "Options" button at the bottom of the table frame and choose "Add Field..."

2. When the dialog box comes up give the field a name, such as "nearroads," choose a field type and a width (called "Precision" in ArcMap). We use the type "Short Integer" because we are only going to store a code indicating whether a block group is inside or outside a buffer.

If you are going to calculate a numeric field, such as Total Population/Area to determine people per square mile, you would choose a field type such as "Double" and set the "Precision" for your estimated width and "Scale" as the number of decimal places (if you choose "Double" as the Type you will see "Scale" appear below the word "Precision") as shown below.

3. After you add a field for the in- and out-of-buffer code you can then "calculate" the value to be placed in the field.

a. Make sure the polygons that are within the buffer are selected, then right-click on the new field "nearroads" and choose "Calculate Values..."

b. Type a "1" in the value box for fivecities_bg.nearroads and click OK.

c. The field "nearroads" is filled in with ones ONLY in the selected records, i.e., the polygons near the roads. The other records have a "0" in the field. If you wanted a different value than zero you could switch the selection (Options->Switch Selection) and with the polygons that are not in the buffer selected, calculate the field again and put in the desired value.

Created by Myounggu Kang on January 25, 2004. Edited by Rhonda Ryznar on January 19, 2006.