Looking at the 795 polls (RealClearPolitics.com) thus far produced shows that there are 55 different pollsters, with the top 4 are Fox, CNN, Gallup, and Rasmussen (See Graph 1). Taking a more detailed look at the top 4 we see that there seems to be a much larger proportional block of those who disapprove of the president in the Rasmussen Reports and Fox News relative to Gallup and CNN (Graph 2). A similar pattern appears in the approval polls numbers.
When comparing polls among the top 4 there is no sense of objectiveness since there is no clarity as to which poll is closer to the objective truth if there is systematic bias in one or more of the pollsters. However, what we can instead do is compare the polls to that of the average within each poll collection phase (say every 50 polls). Removing the time average for every 50 polls we can see a bit more clearly how each pollster is behaving (Graph 5)
is prove to under-reporting both approval and disapproval numbers though Fox is much more likely to under-report approval numbers. Gallup on the other hand is almost the mirror of Fox except that it tends to much more heavily under-report disapproval numbers rather than approval numbers.
Finally, looking at the Rasmussen Reports it is clear that there is something fishy going on. The average disapproval rating report is nearly 7 points above those of other polling locations while the approval rating is under-reported by nearly 2 points. In addition, every poll that is put out by Rasmussen has reported 100% of the times disapproval rates above the average. This is an astounding number since it is clearly highly unlikely.
However, since I am not an expert in polling at all, I am not sure if Rasmussen is doing something legitimate and well known. It is very clear to me from the data however that they are not to be considered equivalent to the other three major polling agencies. Looking at wikipedia, it is well understood that Rasmussen has a different type of polling by selecting individual with a plan to only capture “likely voters”.