The final Des Moines Register poll was just released, showing Donald Trump leading the Republican field in Iowa with 28 percent, Ted Cruz with 23 percent and Marco Rubio with 15 percent. Hillary Clinton was ahead of Bernie Sanders, 45 percent to 42 percent, on the Democratic side. The political world - us included - was eagerly awaiting this survey, as Ann Selzer, who has conducted the Register's polls since the 1988 caucuses, has a very good track record. But just how predictive of the final results have Selzer's polls been? History suggests they're a good indicator of what will...