In Section 11.1, we discuss non-sampling error, which tend to persist even if the sample size gets larger and larger. We then discuss one common non-sampling error: nonresponse. To address this problem, we discuss how to use double sampling to adjust for non-response in the form of callbacks. Then an example is given to illustrate how to compute the estimate and the estimated variance of the estimate. In addition, we provide the formula for selecting the optimal number for call back. In the last part of section 11.1, we discuss how to design surveys to reduce non-response.

In Sections 11.2, the technique of interpenetrating subsample is discussed. An example is used to show this technique which takes into consideration the interviewer effect.

In Section 11.3, we discuss the case when one does not know whether an element belongs to the subpopulation until after it has been sampled and how to estimate the mean and total of this subpopulation. An example is then given to illustrate the method.

*Sampling* by Steven Thompson, 3rd edition

## Objectives

- understand the difference between sampling error and non-sampling error,
- use double sampling to adjust for non-response by callbacks,
- find the optimal allocation for the number of callbacks,
- use interpenetrating subsample technique to take care of interviewer effect, and
- estimate the mean and total over subpopulation