As anyone who follows election polling can tell you, when you survey 1,000 people, the margin of error is plus or minus three percentage points. This roughly means that 95 percent of the time, the survey estimate should be within three percentage points of the true answer.
If 54 percent of people support Hillary Clinton, the survey estimate might be as high as 57 percent or as low as 51 percent, but it is unlikely to be 49 percent. This truism of modern polling, heralded as one of the great success stories of statistics, is included in textbooks and taught in college classes, including our own.
But the real-world margin of error of election polls is not three percentage points. It is about twice as big. CONT.
David Rothschild (Microsoft Research) & Sharad Goel (Stanford), New York Times