House Effects

There appear to be systematic tendencies for some polling firms to favor either Obama or Romney.  Nate Silver calls this tendency the “House Effect” of a particular poll and explains the adjustments that are necessary for a political analyst to fairly use the polling result.   http://fivethirtyeight.blogs.nytimes.com/2012/06/22/calculating-house-effects-of-polling-firms/#more-31415

Are current polls of self-identified registered voters oversamplying Democrats?

http://www.huffingtonpost.com/2012/07/18/mitt-romney-polls_n_1680827.html?utm_hp_ref=politics