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Purpose:

PROB allows you to identify the proportion of districts in an electoral system that has a specified probability of producing a victory for the party of interest. A few examples of the command's uses include:

1. Calculating the proportion of districts with at least a 60 percent probability of going Democratic. To do this, you would type PROB 0.6 1.0;.

2. Calculating the proportion of districts in which no candidate has greater than (say) a 75 percent chance of victory. To do this, you would type PROB 0.25 0.75;.

3. Calculating the proportion of ``marginal" districts, defined as having more than a 25 percent chance of the winner of the last election losing in a hypothetical replication of the same election campaign. To get the proportion of nonmarginal seats, you would type PROB READ 0.75 PTY!vote $<$ datatset;, where PTY!vote is a partisan control variable constructed using the PTY! operator (see Page ).

These examples obviously do not exhaust the possible uses of this command, as it can be used for cases of evaluation, prediction and counterfactual evaluation.