Voluntary Response Sample vs Nonresponse Bias
Both terms below come up in the same part of the course, and students mix them up. Here is each one defined on its own, side by side, so you can see where they part company.
Voluntary response sample
Collecting data and study design
A voluntary response sample is made up of people who choose to respond on their own, such as to an open online poll or a call-in survey.
In a voluntary response sample the participants select themselves, so people with the strongest opinions are overrepresented. For example, an online poll that anyone can answer tends to attract those who feel most strongly rather than a cross-section of everyone. This self-selection usually pushes the results away from the true population value, creating voluntary response bias. Like a convenience sample, it lacks the randomness needed for valid inference.
Nonresponse bias
Collecting data and study design
Nonresponse bias occurs when people chosen for the sample do not respond, and those who do respond differ systematically from those who do not.
Nonresponse bias arises after the sample is chosen, when some selected people do not answer. For example, if a survey reaches a random sample but only the most satisfied customers reply, the results skew positive. The problem is not the missing answers themselves but that non-responders tend to differ from responders. Unlike undercoverage, which happens before contact, nonresponse occurs after people have been selected.