In the case of Clinton, ARG polls are consistently far above the results of other polls. This has been widely remarked upon already. And in the Clinton case, the ARG polls have shown some decline in support in Iowa, while other polls have shown an increase in her support. This is also the case in which ARG exerts a significant influence on the trend estimator. The blue trend line (with ARG included) is well above the red trend estimate which excludes ARG. This was especially true early in 2007 when there were few polls and several from ARG, giving them an extra influence due to lack of non-ARG data. As polling frequency has increased the two trend estimates have converged, but the non-ARG estimate remains a couple of points below the overall trend.
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It is clear that ARG's estimates for Clinton have consistently been out of line with others, and that this has had an effect on my trend estimates, making Clinton appear more competitive in the first half of 2007.
But let's also look at the other candidates. ARG is less consistent in over- or under-estimating Edwards' support. Some ARG polls have put Edwards below trend, but others have him above trend. While ARG has disagreed with other pollsters in individual polls, the effect of ARG on the trend estimate for Edwards is negligible.
On the other hand, ARG has consistently had Obama below the support found in other polls, and well below the trend estimate. Despite this, the effect of ARG on the trend estimates has been small for Obama, with the blue and red trend estimates consistently quite close to one another.
Finally, Richardson has been a bit underestimated by ARG, but again with little influence on the trend estimates.
http://www.pollster.com/blogs/the_effect_of_arg_polling_on_i.php