Going all the way back to 2000, the Review has kept a moving average of the three latest election polls. Back then our error rate was 3 points in the national race (the same as ABC and the Washington Post) and 4 for the governor races, both within the statistical margin of error.
In Iowa this year we had a two point error for Trump, Cruz and Clinton, a four point error for Rubio and a 6 point error for Sanders. We had the Republicans coming in as a statistical tie, which they did but was off by 5 points for the Democrats. Note that the biggest errors were for Rubio and Sanders. Polls can't predict well when people show up who don't normally vote.