Convenience Sample vs Voluntary Response Sample
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.
Convenience sample
Collecting data and study design
A convenience sample includes whichever individuals are easiest to reach, rather than selecting them through any random process.
A convenience sample takes the members who are simplest to access, so it usually fails to represent the population and tends to be biased. For example, surveying only your own friends about a school policy is a convenience sample. Because the selection is not random, there is no way to know the direction or size of the bias, and standard error formulas do not apply. Ease of collection does not make up for a lack of randomness.
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.