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This study assessed the accuracy of telephone and Internet surveys of probability samples and Internet surveys of non-probability samples of American adults by comparing aggregate survey results against benchmarks. The probability sample surveys were consistently more accurate than the non-probability sample surveys, even after post-stratification with demographics. The non-probability sample survey measurements were much more variable in their accuracy, both across measures within a single survey and across surveys with a single measure. Post-stratification improved the overall accuracy of some of the non-probability sample surveys but decreased the overall accuracy of others. Higher completion and response rates of the surveys were associated with less accuracy. Accuracy did not appear to change from 2004/2005 to 2009 for any of the methods, and these conclusions are reinforced by data collected in 2008 as well. These results are consistent with the conclusion that non-probability samples yield data that are neither as accurate as nor more accurate than data obtained from probability samples.
Yeager et al. (Sat,) studied this question.