"FOCUS ON THE BIG PICTURE WHEN POLLS DIFFER" counsels Nate Cohn at the New York Times.
“Individually, [polls] have sometimes felt as chaotic as the news around the world. But together they tell a far clearer story: Mrs. Clinton holds a modest but clear lead heading into the conventions....This straightforward story can get lost in the headlines, which tend to give the most attention to the most surprising results — whether it’s a predicted Clinton landslide or a narrow lead for Mr. Trump in key states. The truth is probably somewhere between those extremes….If Mrs. Clinton leads by 4 points, you should expect polls that show her with a big lead or locked in a tight race, with others clustered around the average. That’s more or less what we saw this past week. It’s a lot like baseball. Even great baseball players go 0 for 4 in a game — or have rough stretches for weeks on end. On the other end might be a few multihit nights with extra-base hits, or a spectacular few weeks. Sometimes, these rough stretches or hot streaks really do indicate changes in the underlying ability of a player. More often, they are just part of the noise inevitable with small samples. Taking more polls is like watching more at-bats, and you need many if you want to be confident about whether a candidate is ahead or tied.”