Reading the Tea Leaves

The latest Mason-Dixon Polls are fairly dramatic in leaving the race open to McCain, but Big Tent Democrat disputes:

The race is not tightening according to Mason Dixon. The Obama lead is growing. A few more examples. In Colorado, Mason Dixon’s 10/28-9 Colorado poll gives Obama a 5 point lead, 49-44. In its previous 9/29-10/1 Colorado poll, Mason Dixon had the race tied, 44-44. In Missouri, Mason Dixon has the race unchanged from its poll last week, a poll NBC commissioned, McCain by 1. In Ohio, Mason Dixon had McCain leading by 1 last week and now has McCain by 2.

I find it somewhat odd to argue that Colorado is showing an Obama lead increase when the prior poll was taken a month ago, when a lot has changed in a month, with McCain hitting bottom mid-October. Ohio and Missouri are fair points. Pennsylvania is perhaps more on-point, showing Obama’s lead at 47-43% with 9% undecided. In another post, Big Tent Democrat points out that a poll from September 16-18 showed Obama by 2. Thjat was right at the end of McCain’s post-convention bump and the start of the economic crisis. There is some tightening here unless you believe Mason-Dixon is on drugs.

Iowa Hawk explains how these polls and the magical margin of error works (Hat Tip: Michelle Malkin.)

This is, for all intents and purposes, how political pollsters compute the mysterious “margin of error,” which has everything to do (and only to do) with pure mathematical sampling error. If you look at the formula above and round it just a smidge, you get a simple rule of thumb for the margin of error of a sampled probability:

Margin of Error = 1 / sqrt(n)

So if the sample size is 400, the margin of error is 1/20 = 5%; if the sample size is 625 the margin of error is 1/25 = 4%; if the sample size is 1000, it’s about 3%.

Great, but if your sample is not representative of the population, your margin of error doesn’t matter. So the question ultimately how undecideds unbreak and which polls figured out who would be voting. Everyone work as hard as you can. Win or lose, let’s do it fighting.