Another piece of housekeeping and one of Eric Dienstfrey's final contributions to Pollster.com. We have produced a new chart that includes only polls that report the Obama job rating among all adults. The original Obama job rating chart that includes all surveys remains in place; this new chart adds a new way of tracking the trends.
We have discussed some of the challenges posed to our charts on measures like the Obama job performance rating by pollsters whose results show big "house effects" (consistent differences when compared to other pollsters). Our philosophy has always been to try to include all polls that claim to produce representative samples -- even those based on more controversial methods such as automated polls or those that survey respondents over the internet using pre-recruited panels -- to make it possible to use our interactive chart features to compare and contrast different surveys.
The problem is that if big house effects occur, the trend lines can sometimes display phantom trends when polls with consistently different results are more frequent. This issue crops up most often in the "nose" of the trend line, which moves around more than the rest of the line as we add new polls to our database. The Rasmussen Reports surveys appear to be a big problem in this respect, mostly because they are far more numerous. However, if you use your mouse to click on Obama job ratings that tend to be higher or lower than other polls, you will also see pollsters with similar house effects that poll less often.
We offer the new all-adult-sample-only charts as one means of reducing the potential for "phantom" trends, though we have other potential improvements in the works. Please let us know what you think.
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