“Probabilizing” uncertainty in the Brazilian Presidential Election

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The following figure shows the probability distributions of vote intentions for the main candidates after distributing the stock of undecided voters. As Marina (PSB) is getting back to her track, a question that comes to light is whether Dilma will get more votes than the sum of the others, and what is the probability that she will take it all without a runoff.

Based on the latest polls and simulations (100k) her chances of avoiding a runoff are not that great. There is a 98% chance of seeing a runoff between Dilma and Marina as indicated in the bottom line in graph below. However, if she gains this (most likely) proportion of the undecided voters and “steal” a few more from the other candidates, then that will increase her probability of winning even in the first round.


To make it more concrete, in other to avoid a runoff with Marina, Dilma needs to hold a plurality of the late deciders (which is theoretically likely as the undecideds tend to preserve the status quo) and soak up around 3% from the other candidates. Not much but it would end up being a 50/50 chance then.


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