|
If you pick a sample that is typical of the population, you can expect that if you took the poll 100 times (of 100 voters each time), on average the result will be extremely clost to the result you'd get if you polled every person in the country. But if you only take the poll once, you'll see some difference between the poll result and the national "correct" result, but it's equally likely to be off in either direction.
Based purely on the sample size, the margin of error measures how likely it is that you just randomly happened to pick too many Bush supporters, for example. If you graph the probability of being off by a certain amount (because your sample didn't represent the population), you get a bell-curve shaped graph (at least for samples larger than 20 or so).
The +-3% numbers are the range where it's 95% likely that the sample was within 3% of the result you'd get if you surveyed the entire population. This is computed purely based on the chance of getting a questionable sample.
You can still be off for reasons other than the chance of getting a bad sample - you can get a greater number of deliberate refusals from one party than the other, the questions could be biased, you could be ignoring a group like cell-phone users, and so forth.
The polls try to make up for the sources of bias by sampling Republicans, Democrats, and independents seperately and weighting the results by how many of each are registered and how likely they are to vote. If Republican turnout isn't higher like it normally is, this would be another source of error, one that people keep mentioning for Gallup polls that oversample Republicans.
(It's also possible that some polls are reporting margins of error that are themselves guesses and not based on the sample size issue discussed above, but this wouldn't be standard statistical practice.)
|