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“Forecasts,” snapshots, and predictions, 2012

June 27th, 2012, 2:51pm by Sam Wang

Over at Matt Dickinson’s Presidential Power, there’s a discussion between him, Nate Silver, and others. I weighed in on the confusion between poll snapshots (what all aggregators do, including Nate Silver’s “forecasts”) and true predictions (what political scientists do). Here’s part of what I wrote…

Basically, I think the term “prediction” is used too loosely. I’ll make an analogy to weather forecasting. Weather forecasting begins from immediate measures such as temperature, wind speed, and other current conditions. Adding likely future changes, whether on a short time scale (prevailing wind) or a long time scale (climate modeling) can add useful information to inform the future.

When you’re watching a hurricane, what you want is the person with current conditions and short-term trends. However, to learn about the next hurricane, different expertise comes in.

(1) What a poll analyst like Nate Silver or myself starts from is a snapshot of current polling conditions, sometimes with uncertainties added into the mix. In the analogy, we are weathermen.

(2) Political scientists like Ray Fair and successors provide a prediction of a future event. In the analogy, he is the climatologist.

These approaches often overlap, though it is not acknowledged explicitly. Combining them produces a third category (3), in which current conditions (the snapshot) and likely future changes can both inform a true prediction for a current race.

For example, Silver makes efforts to draw upon (2) a bit, and calls his hybrid calculation a prediction. Calling it a prediction satisfies his readers’ desire for one. He also provides very interesting ongoing commentary. All of this is legitimate, but over at Dickinson’s blog he is rather negative about approach (2). However, the weatherman should take care in castigating the climatologist.

“Forecast” is a bit of a misnomer when applied to poll aggregation, in the sense that weather moves in predictable patterns, and therefore one can forecast that a storm in Chicago will usually end up over Ohio in the future. But for polls, the components of a “forecast” mainly consist of adding a dose of uncertainty, i.e. random drift. Think of poll aggregators as weather rocks.

A hybrid approach potentially does not add information to understanding where the Presidential race stands today — or in November. I would argue that it tends to conceal what (1) and (2) above can each tell us separately — and approach (2) is open to debate, as seen over at Presidential Power. Here at the Princeton Election Consortium I have in the past provided (1), a pure Snapshot of Polls. Soon we hope to unveil the Snapshot for 2012.

However, a hybrid approach (3) is very useful for assessing individual races where less data are available, such as House and Senate races. This is an important practical application. If done cleanly, one could imagine using a variety of variables, including past trends and campaign spending, to inform a true prediction. That’s the real value added by aggregators who claim to make predictions.

Tags: 2008 Election · 2012 Election

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