Thursday, March 08, 2007

Bush Approval: NBC/WSJ 35%, Trend at 33.4%

The new NBC/Wall Street Journal poll, taken 3/2-5/07, finds approval of President Bush at 35% and disapproval at 60%. Those results are identical to the previous NBC/WSJ poll taken 1/17-20/07. With the new poll my estimate of approval (the blue line) now stands at 33.4%.

For a detailed explanation of the graphs here, see this post. The goal of these plots is to place each poll in the context of the estimated trend, other polls, deviations from the trend, and the variability of these estimates over time.

The most recent six polls are shown below, along with the trend estimate. To see the entire set of 19 recent polls visit the approval page at here.

The deviations from the trend give a sense of when polls are within the expected range of values we would expect due to random sampling and non-sampling errors. The 95% confidence interval plotted here shows the region within which 95% of polls fall. Polls outside that range are "outliers" and should be viewed as unexpectedly far from trend. There are various reasons why a poll might fall outside this range, and a single outlier should not be considered evidence of "bad" polling. But such polls should be interpreted skeptically in terms of where approval actually stands. The trend estimate remains the best estimate of approval at any point in time.

The trend estimator itself is subject to variability due to the timing of polls by various organizations. The plot below shows the range of estimated trends based on 20,000 bootstrap replications of the trend estimator. This is a conservative estimate of how variable the trend estimator is.

Each new poll at the end of the time series exerts significant influence on the latest estimated trend. This estimate will vary as new polls come in. The plot below shows the estimates for the last 20 "latest polls." The blue trend line is our best estimator of the path of approval after looking over all the available data. The variation in the red dots shows how much this has varied over the past 20 polls.