Equation: Calculating Ballot Bungles Is All About the P-Value

Tuesday, 16 November 2010 13:00

Equation: Calculating Ballot Bungles Is All About the P-Value

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Elections have a dirty secret: Counting lots of ballots with absolute precision is impossible. Those infamous hanging chads from a decade ago are just one of the many glitches that can—and do—arise. Several states require a super- careful hand count in select precincts to double-check close results, and they nearly always find miscounted ballots. A few such slipups don’t matter, but multiple mistakes will alter the election results. Most laws leave the determination of the recount threshold to the discretion of registrars.

But not California—at least not since earlier this year, when the state assembly passed a bill piloting a new method to make sure the vote isn’t rocking a little too hard. The formula comes from UC Berkeley statistician Philip Stark; he uses the error rate from audited precincts to calculate a key statistical number called theP-value. Election auditors already calculate the number of errors in any given precinct; the P-value helps them determine whether that error rate means the results are wrong. A low P-value means everything is copacetic: The purported winner is probably the one who indeed got the most votes. If you get a high value? Maybe hold off on those balloon drops. Photo: Daniel Salo

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P The probability that you’d see the current error rate if the winner didn’t really win. If P is higher than about 10 percent, the result might well be incorrect, so keep double-counting more precincts.

n
?
i=1

The product of multiplying results for all the audited precincts (n). For each precinct, calculate the maximum possible error (U) and . Then multiply all those ratios together.