In a national field experiment, the same questionnaires were administered simultaneously by RDD telephone interviewing, by the Internet with a probability sample, and by the Internet with a non-probability sample of people who volunteered to do surveys for money. The probability samples were more representative of the nation than the non-probability sample in terms of demographics and electoral participation, even after weighting. The non-probability sample was biased toward being highly engaged in and knowledgeable about the survey’s topic (politics). The telephone data manifested more random measurement error, more survey satisficing, and more social desirability response bias than did the Internet data, and the probability Internet sample manifested more random error and satisficing than did the volunteer Internet sample. Practice at completing surveys increased reporting accuracy among the probability Internet sample, and deciding only to do surveys of on topics of personal interest enhanced reporting accuracy in the non-probability Internet sample. Thus, the non-probability Internet method yielded the most accurate self-reports from the most biased sample, while the probability Internet sample manifested the optimal combination of sample composition accuracy and self-report accuracy. In a laboratory experiment, respondents were randomly assigned to answer questions either on a computer or over an intercom system with an interviewer; the former respondents manifested higher concurrent validity, less survey satisficing, and less social desirability response bias. These results suggest that Internet data collection from a probability sample yields more accurate results than do telephone interviewing and Internet data collection from non-probability samples.