When you calculate the percentage of partial columns (such as population columns that cite income, gender, age, ethnic background, or religious affiliation) you have two calculation methods available. One method calculates the percentage by column so that the sum of all of the entries in every column would be 100%. Another method calculates the percentage by row based on your selected row entry (or sum of entries) so that each percentage entry in the row is calculated based on that row (or sum of entries). For example, in the following table:
| A | B | C |
|---|---|---|
1 |
1 |
1 |
2 |
2 |
2 |
The Column method determines the percentages of entries in Column A:
Percentage (A1)=A1/(A1 + A2) x 100%; Percentage (A2)=A2/(A1 + A2) x 100%The Row method determines the percentage for entries A1 and B1 based on C1 as a total column:
Percentage (A1) = A1/C1 x 100%; Percentage (B1) = B1/C1 x 100%Redistricting using the Row Method
To redistrict using the row method:
- Open a new redistrict window with a file containing columns with partial percentage entries in it. (We used the USA.TAB file that ships in the Introductory Data on the MapInfo Pro DVD.)
- For the Row or Column method, select the field to calculate the percentage for from the Fields to Browse list.
- For the Row method, select the base field (or sum of fields for an expression) for your calculation from the Expression drop-down list.
- Select one of these two options:
- Column Total - Use this method to calculate the partial percentage of the selected column based on the sum of the other column entries. This is the original calculation method.
- (Field or Expression) for each row - Use this method to calculate the partial percentage of the selected column based on a particular field or expression based on that row of data.
When you select the Expression option at the bottom of the list, the Expression dialog box displays. Complete your expression and click OK to return to the New Redistrict window. This is the new calculation method.
Note: When you create an expression for this operation, the only valid operation is a sum.
- Click OK to calculate the percentage you selected.
For example: To calculate the percentage of the urban population of each state based on the total urban population for all states, select the Pop_Urban_2000 field in the Fields to Browse list and choose the Column Total option in the Calculate Percentage based on section. MapInfo Pro calculates the percentage for the urban population on each state (Alabama Pop_Urban_2000) based on the total urban population for all states (Pop_Urban_2000).
To calculate the percentage of the urban population for each state based on the total population of that state, choose the Pop_Urban_2000 field in the Fields to Browse list and in the (Field or Expression) for each row option, select the Pop_2000 field. MapInfo Pro calculates the percentages based on each state's population data only. So for Alabama, the calculation would be the urban population row entry (Alabama Pop_Urban_2000) based on the data in the total population column for the same state (the Alabama Pop_2000 column). This calculation would be performed for all states in the table.
Creating New Districts
If you do not have any established districts in your table, you can create them as you go during the redistricting session. You should first, however, prepare your table to store the new district names by adding an empty column to your table. To do so, on the Table tab, in the Maintenance group, click Table, and Modify Structure.
With that done, on the HOME tab, in the Windows group, click New Document, and click Redistricter from the list, to open the New Redistrict Window dialog box. Choose your table from the drop-down list. Choose the district field from its drop-down list. The district field is the empty field you created in your table to hold the district names.
A powerful feature of redistricting is the ability to calculate net total values from the data in your table. Any numeric field can be aggregated by sum or percent during the redistricting session. The Available fields list shows you what calculations are possible for the given table. Choose from this list and click Add >> to move the expression to the Fields to Browse list. Only those items listed in the Fields to Browse list will be acted upon and displayed in the Districts Browser.
You can reorder the fields using the Up and Down buttons to list them in a certain order in the Browser. When you are satisfied with the choices in the dialog box, click OK. MapInfo Pro performs the redistricting operation.
When processing is finished, MapInfo Pro displays a map of your table with a single color and a Districts Browser with a placeholder for a district entry. This is because you told MapInfo Pro to carry out the redistricting on a column with an empty field.
To create another district, make sure the Districts Browser is active so that the Redistricter command list is available on the MAP tab. Click Add District. The new district displays in the District Browser with the name DistrictN, where N is the next higher number. Continue to add districts until you feel you have enough to begin reassigning objects to them. Enlarge the Browser, if necessary, to see the new district records.
To begin building your districts, make one of them the target district. Next, select map objects in the Map window to add objects to the new district. Notice how the values in the target district increase as the object(s) are selected.
At this point the objects are only temporarily assigned to the district. When you have made all the additions to the district that you want for the moment, make the Districts Browser active and on the Redistrict command list, clickAssign District to make the assignment permanent. You cannot build another district until you complete the current one.
You must save the table at the end of the redistricting session to retain the new districts.
Continue to add and assign objects to districts until you have created as many districts as you need. You see the geographic extent of each color-coded district in the map window. You can also see the results of the expression calculations in the Browser showing the new total values for each district.
Now you can reassign districts to more equitably distribute resources, to include or exclude certain information, whatever your needs call for.
Reassigning Existing Districts
If you already have districts in your table, or have a column that contains values that are shared among the records, you can use those as the basis for your districts. When you activate the Redistricter, these values display as color-coded districts on a map.
For example, choose a column that contains information that will make up groups of more than one record each, such as sales reps, postal codes, area codes, states. If you choose a column containing unique values like customer names you will create districts that only contain one record each, not very useful for creating districts.
On the HOME tab, in the Windows group, click New Document, and clickRedistricter from the list, to start the redistricting session. In the Redistrict dialog box, specify your table and field that contains the district names. From the Available Fields list, choose the fields you want displayed during redistricting. Reorder the fields as necessary. The fields will display in the Districts Browser in the order specified in the Fields to Browse list. Click OK. MapInfo Pro creates an individual value theme and the Districts Browser.
Now to begin the actual redistricting, or load-balancing, set the district that you want to add objects to as the target district. Select the map object(s) and watch the data change in the Districts Browser. You can select objects from more than one district and assign them to the same target district. Simply Shift-click each additional map object after the first one.
Before realignment, children under 4, in the column Sum(Child_0_4), were unevenly distributed among the districts.
As you choose map objects, the Browser immediately reflects the changes among the districts with new counts per district and new totals based on expressions you specified in the Redistrict dialog box, such as the sum or percent of a column value.
When you are satisfied with the reassigned objects for the target district, on the MAP tab, in the Options group, click Redistricter, and Assign District to make the changes permanent. You can choose a different target object and add other map objects to it. Continue until you are satisfied with the new distribution of map objects.