The Brown Bear & The National Margin (Part One)

In this post the Brown Bear explains how he calculates the Bush vs. Kerry portion of the National Margin. In Part Two the Brown Bear will explain the Job Performance part of the National Margin.

As an example let's take the Time & Newsweek polls that were released last weekend. Time reported Bush 51% Kerry 46%, while Newsweek reported Bush 48% Kerry 46%. The first thing the Brown Bear does is calculate the poll margin for each poll: Time +5%, Newsweek +2%. (A Kerry lead would be a negative number.)

The next thing the Brown Bear looks for is the days the polls were taken. The Time poll was conducted from Oct. 19 - 21, while the Newsweek poll was conducted from Oct. 21 - 22. The Brown Bear then determines the midpoint of the range of dates for each poll: Time, Oct. 20, Newsweek Oct. 21 & a half. The Brown Bear then calculates the Age of the poll simply by counting the number of days from the end of August to the polls midpoint: Time 50, Newsweek 51.5. Note that more recent polls have higher ages and older polls have lower ages.

The Brown Bear then calculates the weight to assign each poll from the age of the poll. The Brown Bear takes the quantity, the seventh root of two (approximately 1.1), and raises it to the power of the poll's age. The weight of the Time poll is 141.323 and the weight of the Newsweek poll is 163.953. Note that the more recent poll, Newsweek, has the heavier weight.

The Brown Bear then multiplies the poll margin by the poll weight and adds them. For Time & Newsweek, the sum is 10.35. The Brown Bear then adds the poll weights. For Time & Newsweek this sum is 305.276. The Brown Bear then divides the sum of the poll weights into the previous sum to get the National Poll Margin. Just considering Time & Newsweek, this margin is 3.39%. Note that a straight average of the two polls would be 3.5%. This average is lower because the Newsweek poll is more recent and thus weighted more heavily.

Part Two of this description will describe how the Brown Bear uses President Bush's Job Performance rating in combination with this one to obtain the National Margin.