Reply #10: The typical confidence interval is 95% [View All]

[Silent3]

The confidence interval, unlike the margin of error (MoE), is hardly ever explicitly stated.

If a poll says Obama is favored by 52% of voters, with a 3% margin of MoE, that means there's a 95% chance that actual voter preference is somewhere in the range 49%-55%. In one out of 20 such polls the real value will fall outside of the MoE.

Error distribution is typically a bell curve, so the most likely real values are closest to the reported value. If you see a poll you don't like, say Obama at 42% and McCain at 47%, it's possible, but not very likely, however, that this really means 45/44 in favor of Obama.

The big problem with interpreting polls and the margins of error is that the stated MoE is in some ways just a mathematical abstraction which doesn't necessarily capture the full range of reasons to doubt the accuracy of a poll, like bad polling methodology or bad demographic weighting of the raw polling data.