'Missing' responses

The first two types of non-sampling error are caused by failure to obtain information from some members of the target population.

Coverage error

Coverage error occurs when the sample is not selected from the target population, but from only part of the target population. As a result, the estimates that are obtained do not describe the whole target population — only a subgroup of it.

Non-response error

In many surveys, some selected individuals do not respond. This may be caused by ...

If non-response is related to the questions being asked, estimates from the survey are likely to be biased.