Were the polls wrong? It's a question asked after every election. Sometimes, as in 1948, the answer seems as obvious as the answer to the question, "Why did Custer lose at Little Bighorn?" Sometimes the answer is less obvious, as it is this year. "The polls were skewed toward the Democrats," writes Nate Silver, who as proprietor of FiveThirtyEight has earned the distinction of being the nation's most assiduous polling analyst. Silver gives short shrift to partisans -- Democrats this year, Republicans in 2012 -- who complained that polls were systematically biased against their side. The skew varies unpredictably, he...