Mark Blumenthal | September 19, 2008
Topics: 2008 , Charts , Map , Status Update , Trend lines
Another day, another 37 mew statewide polls (as of this writing) logged into the Pollster.com database. Today's batch managed to change classifications in several states into the toss-up column. Specifically, the new Big Ten polls in Minnesota, Wisconsin, and Pennsylvania helped tip the balance to move those states from lean Obama to toss-up. Two new polls in New Jersey helped move that state from strong to lean Obama.
When we returned from the Republican convention, our map classifications showed Obama with 260 electoral votes, McCain with 179 and 99 in the toss-up category. Since then, Obama's total dropped to 202, McCain's grew to 208 and the electoral vote total of those states currently rated as toss-up states has swelled to 128.
As of today the trendline for the latest national surveys shows a modest rebound for Obama over the last week. Our current national estimate shows Obama leading by just over two percentage points (47.2% to 44.9%). About a week ago, McCain had moved slightly ahead.
Any such trend is not obvious in the state trends, which look like as much of a dead heat as they have since we started running them this summer. But keep in mind that our state level classifications are based on state-level polling only, and the trend lines in our charts and the estimates they produce are inherently conservative, in that they require more than one new poll before the trendline moves significantly. As such, our current delegate count probably reflects where the national trends were about a week ago.
The way we classify states probably deserves some explanation, since our traffic has grown considerably and, if the questions in my inbox and any guide, many of you assume that our classifications are subjective (as they are on many other sites). To be clear: Our process is entirely empirical and automated. We input new polls and the system draws loess regression trendlines for each state. The end-point of the trendline serves as our estimate (analogous to the "averages" you see on other poll aggregating web sites). We then calculate confidence intervals (margin of error) around each estimate based on the average sample size for the polls in each state (more details in our FAQ).
Thus, the classification is automated and depends entirely on the size of the margin separating the candidates. The classifications will sometimes be slightly inconsistent from state to state, because the polls in some states (such as Pennsylvania and Ohio) use bigger sample sizes on average than others (such as West Virginia and Vermont ).
Also, as discussed earlier in the week, the trend estimates in smaller states with fewer available polls tend to be more sensitive to the latest new polls. As such, changes in classification in states like West Virginia and Montana (where polls are rare) may occur on the basis of fewer new polls than in states like Ohio or Pennsylvania (where they are far more frequent).