538 did some good analysis and explinations about this exact issue, but the articles that I can quickly google are from a while ago [0][1]. You can look over th

538 did some good analysis and explinations about this exact issue, but the articles that I can quickly google are from a while ago [0][1]. You can look over their analysis of different pollsters that they use and get a feeling for what 538 thinks is bias [2]. Despite some negative press in some circles, they tend to do a very good job of making sense of the maddness that is polling and they are very upfront about their own biases and issues. [0] https://fivethirtyeight.com/features/study-excluding-cellphones-introduces/ https://fivethirtyeight.com/features/study-excluding-cellpho... [1] https://fivethirtyeight.com/tag/polling-bias/ https://fivethirtyeight.com/tag/polling-bias/ [2] https://projects.fivethirtyeight.com/pollster-ratings/ https://projects.fivethirtyeight.com/pollster-ratings/