Day-of-Week Effect in Gallup Daily? ... and Rasmussen.

Day-of-Week Effect in Gallup Daily?

I received some interesting charts this afternoon from Harrison Hickman and Ben Margolis, both of the Democratic firm Global Strategy Group that polled for John Edwards until he withdrew from the race (full disclosure: Hickman was my employer longer ago than either of us wants to admit). When Hickman and Margolis plotted the Gallup Daily results for the Obama-Clinton race, they noticed an impressively consistent pattern by day of the week. In data released during February and March, Obama typically does best on three-day samples that end on Saturday (combining interviews from Thursday, Friday and Saturday), while Clinton typically does best on samples ending on Tuesday or Wednesday night (covering Sunday through Wednesday).

The chart below shows the pattern in the Clinton lead (Clinton minus Obama) for each of the last nine weeks:

Hickman and Margolis sent a second chart which shows the average for each daily release across the nine-week period. On average, the results have shown a more than three point shift from Sunday-Monday-Tuesday (Clinton ahead by 1.3) to Thursday-Friday-Saturday (Obama ahead by 2.6).

So, given this pattern, do not be surprised if the ten-point Obama lead reported yesterday (based on a interviews from Thursday through Saturday) narrows a bit more by mid-week from the eight -point lead reported today. The results also suggest that we would get a less "volatile" sense of the race by looking at a seven-day rolling average that eliminates the apparent day-of-week effect.

The more interesting question is, why are we seeing this pattern? It seems counterintuitive, at least at first blush. I would have expected Obama's supporters to be harder to reach on weekends, given that they tend to be younger than Clinton supporters. Back in December, Obama's supporters seemed to be harder to reach around Thanksgiving and the Christmas holiday. Unless I am missing something obvious, this pattern is different.

Any thoughts? Theories?

UPDATE: Hold the, er, phone. I sent Hickman and Margolis a spreadsheet with the Rasmussen Reports daily tracking and they generated comparable charts. Oddly enough, the pattern is almost the mirror opposite of Gallup. On the Rasmussen automated tracking (which reports a four-day rolling averge), Obama does consistently better in the middle of the week, Clinton better on the weekends. Here is the chart of each week:

And here are the average values for each daily release:

I will have a chance to add more thoughts later, but I think the bottom line is that the conflict of the patterns shown by the two pollsters suggests that the apparent day-of-week effects are about differences in poll methodology, not a real variation in voter preferences over the course of the week.

-- Mark Blumenthal

FOR CHARTS and DETAILS, GO TO ---> http://www.pollster.com/blogs/day_of_week_effect_in_gallup_d.php