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Commentary

Polling error | June 3rd, 2008
Reading over the polls, I noticed the following comment on CNN:

	The new USA Today/Gallup survey shows [Obama] with a 5 point advantage over McCain among likely voters, 49 percent to 44 percent. That margin is just outside the poll's 4 point margin of error, meaning Obama appears to hold a slight advantage over the Arizona senator with five months remaining until voters weigh in at the polls.

The margin of error for the support of a single candidate is 4 percent; the difference does not have the same error since the proportion of people who support McCain is negatively correlated with Obama's support. That is, if Obama's support is actually 2 percent lower, it is likely McCain's support is about 2 percent higher. So for the difference to be truly significant, we have to see a larger point advantage.