House effects refer to the internal dynamics of how polls are constructed and conducted by major pollsters that can cause polls from a particular organization to favor one candidate over another. House effects aren’t necessarily intentional. Most legitimate polling organizations depend for their existence on getting their poll numbers right. Never-the-less, some polls tend to favor Democrats and others tend to favor Republicans, if ever so slightly. That can make a big difference in the results. When you see poll results you need to take into account the House Effects. Here is a chart that will help you. Print it and keep it with you. If you can’t see the chart, go here: http://www.huffingtonpost.com/simon-jackman/house-effects-by-back-by-_b_2007907.html
As you can see from the chart, Zogby Internet polls tend to favor Obama by just a little less than 2 percent. Gallup polls tend to favor Romney by a similar percent. So, you should subtract 2 percent from Obama’s rating in Zogby polls and add 2% to Obama’s rating in Gallup polls.