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As of this writing, Leo suggests that the Democrats have a 51% chance of retaining the Senate in the 2014 elections. Now, probabilities are subtle things, and some commentators are prone to report from an estimate like this that "Leo predicts the Democrats will win the Senate in 2014". Of course, that would still be a risky bet to make, "essentially the same as a coin flip" as the Leo website says. As long as the probability isn't 0% or 100%, the actual outcome is always in doubt, dependent on factors we can't measure and over which we have no control. (In other words, luck.) The Upshot does a lovely job of demonstrating this variability with a feature that spins roulette wheels, loaded according to the data (mainly polls) for each race, and simulates one possible outcome.

This is a fantastic way to demonstrate the inherent variability in any statistical forecast, and it ties in with the underlying methodology as well. (For the statisticians out there, those wheels don't spin independently; correlations between individual races are taken into account.) Spin those wheels hundreds or thousands of times, count the number of times Democrats or Republicans win, and you have an estimate of the overall probability each party wins. Nicely done!