Kudos to Gallup for disclosing the process and perils of its likely voter model, but as Alan Abramowitz has noted at Pollster.com, something about the new USA Today/Gallup poll showing John McCain 4 points ahead among likely voters — but 3 points behind among registered voters — doesn’t quite sit right:
How do you get from a 47-44 Obama lead among RVs to a 49-45 McCain lead among LVs?
A few quick calculations shows how. You have 900 RVs and 791 LVs, so that means that among your 109 UVs (that’s unlikely voters according to Gallup) Obama leads McCain by a whopping 61% to 7%.
Putting it another way, according to Gallup 16% of registered Obama supporters are unlikely to vote compared with only 2% of registered McCain supporters.
Whatever one thinks about likely voter models in general, the mathematics of this particular implementation defy credulity. Although, we should probably wait for USA Today to release its crosstabs so we can make sure there wasn’t a typographical error of some kind in the write-up.
Also, this is a good time to mention Robert Erikson’s critique of the extra volatility introduced by Gallup’s likely voter model in past election cycles.