Answer:
a. No, Returned questionnaires can't be regarded as a random sample of households.
b. Non Response Bias
Explanation:
a. Among the 500 households only 80 responses to the survey. This type of sample can't be regarded as a random sample. Because it is possible that the question asked to people contain any embarrassing information that peoples refuse to answer the questionnaire.
b. This type of bias is known as Non-Response Bias.
Further, Non Response bias can be considered as, In conducting a survey some people did not respond to our survey, this sometimes affects our survey result very much.
For Example: It can happen that some people may refuse to participate in a survey, as the question asked to people contain personal detail or illegal activities or asking any embarrassing information, so people refused to participate in the survey. This non-response causes the results of the survey to be biased.