This study explored whether probability sample telephone survey data and data from nonprobability sample Internet surveys yielded similar results regarding intent to complete the 2010Census form and actual completion of the form, the correlates of these variables, and changes in these variables and their correlates over time. Using data collected between January and April, 2010, the telephone samples were more demographically representative of the nation’s population than were the Internet samples after post-stratification. Furthermore, the distributions of opinions and behaviors were often significantly and substantially different across the two data streams, as were relations between the variables and changes over time in the variables. Thus, research conclusions would often be different depending on which data stream was used. Because the telephone data collection methodology rests on well-established theory of probability sampling and produced the most demographically representative samples, the substantive results yielded by these data may also be more accurate than the substantive results generated with the non-probability sample Internet data.