General Discussion
In reply to the discussion: Reuters: Democratic support for Hillary drops 15 points since February [View all]DemocratSinceBirth
(99,710 posts)Surveys of the general population that rely only on the internet can be subject to significant biases resulting from undercoverage and nonresponse. Not everyone in the U.S. has access to the internet and there are significant demographic differences between those who do have access and those who do not. People with lower incomes, less education, living in rural areas or age 65 and older are underrepresented among internet users and those with high-speed internet access (see the Pew Research Internet Project for the latest trends).
There is also is no systematic way to collect a traditional probability sample of the general population using the internet. There is no national list of email addresses from which people could be sampled, and there is no standard convention for email addresses, as there is for phone numbers, that would allow random sampling. Internet surveys of the general public must thus first contact people by another method, such as through the mail or by phone, and ask them to complete the survey online.
http://www.people-press.org/methodology/collecting-survey-data/internet-surveys/