How to Build a Model

First Step — Select a Time Period to Model

The first step is to select a data type and time period so that you can see what the actual migration was and what statistics are available to build your model with. First, find the Use data by list and select either County, to use statistics gathered for each county in the United States, or State, to use statistics gathered for each state. Then, find the Time Period list, and select one of the available time periods. The statistics database for that time period will be downloaded, along with a map of the actual migration during that time period, labelled Actual Migration

Second Step — Select Variables

A number of statistics have been gathered for each region over the course of your selected time period. To build your model of migration, you will need to select which ones you believe were important factors in the migration of that time period. To get a list of variables from which to choose, press Select Variables.

To select one of the variables from the list, simply click on it. This will deselect all your other variables, however. To select, or deselect, a variable without changing the status of all the other variables, hold down the Control key as you click the variable. To select an entire range of variables, click on the beginning of the range, then hold the Shift key, and click on the end of the range. When you have selected all the variables you want, press the OK button.

Third Step — Set Values of the Variables

The variables you select will appear in the Variables list on the left of the screen. Underneath the name of the variable, there is a 'slider' control, labelled Predicted Best Value. This control allows you to set a value that you think was most attractive to migrants during that decade. The slider starts at the mean value of that variable. The value of the current setting appears above the slider control area.

To change the setting, simply drag the the thumb piece of the slider left or right. Alternatively, you can click on the track at the position you want to move the thumb piece to. In either case, the updated value of the variable will be displayed above the slider.

If you have selected more than one variable, a slider control labeled Importance will appear to the right of the variable name. This control lets you adjust the relative importance of each variable in your model. You may make a variable less important by dragging the slider to the left or more important by dragging it to the right.

Fourth Step — Run Your Model

To see the migration based on your model, press the Run Model button. After some calculation, a map will appear in the lower right with the migration that would have occurred during that decade if your model was what actually affected migration.

At the top of the Modelled Migration map, a value labeled Match appears. This number indicates the correlation of the migration between the Actual Migration and the Modelled Migration maps. The Match number ranges from -1 to 1, where -1 indicates that the maps are completely opposite and 1 indicates a perfect match. Generally, a match is considered good if it is greater than 0.3.

Fifth Step — Improve and Test Your Model

Look at the differences between the Actual Migration and the Modelled Migration maps. Think about likely causes for those differences. Then adjust the Predicted Best Value and Importance sliders. Run the model and readjust the sliders until you get as good a match as possible. Also, try other variables that you think may have an impact.

You can also compare a model between different time periods. After a model is built, you can select another time period in which to run your model.

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