Surveys of People

Data obtained from surveys of people is often inaccurate because of the way the surveys are designed and carried out.

Selection Bias
Selection bias is when the sample obtained by a survey fails to represent the population. There are three main ways this can occur.

Voluntary Response - When sample members are people that have volunteered. People who have stronger opinions about something are more likely to volunteer those opinions, so this kind of sample tends to overrepresent those people.

Nonresponse – When people chosen for the survey don’t participate in it. If the kind of people who don’t respond are the type of people with a certain opinion, then the results will be biased.
For example, if you select 1000 people at random from the phone book, ring them up, and ask them “do you respond to telephone polls?” the nonresponse bias will make your results completely meaningless.

Undercoverage – When certain subsets of a population are, for whatever reason, not represented in a sample.

Response Bias
Response bias is when the survey itself affects the kind of responses received. There are two main ways this can occur.

Question Wording – When survey questions are asked in a way that favours certain responses.

Social Desirability – When people don’t give true answers because they are reluctant to admit to unsavoury attitudes or activities, or illegal activities.