At this moment, PEC’s probability of D+I is 49%. Yesterday it was 52%. Obviously everything is different, a volte-face. Right? Um…
For a refreshingly accurate and insightful look at how to think about knife-edge probabilities, Mark Mellman has written an excellent article for The Hill.
[Some people] exchange the numbers for words. “Republicans,” they say, “are likely to take over the Senate.” Well, that depends on what “likely” means to you — and research tells us it means different things to different people. For some people, likely is akin to 90 percent; for others, it could be 60 percent. Thinking about the Senate forecasts this way results in a loss of information and precision.
What [Senate forecasts] may mean is that it is a bit more likely that the GOP wins control of the Senate this cycle than that Democrats keep it.
But consider the classic illustration of probability: coin flipping. If you flipped a coin a thousand times, it should come out heads about 500 times and tails 500 times. If you weren’t actually counting, you couldn’t possibly notice the difference between that 50-50 chance and, say, 510 heads and 490 tails — equivalent to a 51 percent probability. Which is to say, the difference is imperceptible. You probably wouldn’t even notice 590 heads and 410 tails as being particularly off the 50-50 mark.
It’s good – just go read the whole thing.
For this reason, last week’s fuss over the difference between PEC and other forecasters was overblown. Now that we’re microscopically under 50% for the Democrats, I am steeling myself for a round of “aha, now you agree!” However, anyone who has any understanding of the situation should know that control of the Senate is up in the air. All we know is where activists should go to make a difference: Iowa, Colorado, and a shifting pattern of states – today, Kentucky and Georgia.
Also, I estimate that any poll-based estimate, whether at other sites or at PEC, has at least a 15% uncertainty, i.e. the true probability today is between 35% and 65%. Of course, that does not change the fact that the overall trend over the last month has been toward the GOP.
Let me finish by quoting Alan Koczela from yesterday’s thread: “There are worrying signs, even in PEC’s model. The meta-margin tanked in late September and is bouncing around R territory. And, the likelihood D+I number is below 50%….these should be worrying signs for Democrats. No need to panic, but, if you’re not concerned, you’re a fool.” Also see my reply to him.