Estimation

The aim of sampling is usually to estimate one or more population values (parameters) from a sample. Because we do not have complete information about the population, the estimate is unlikely to be exactly same as the value that we are estimating, so we call the difference the error in the estimate. There are different kinds of error.

Sampling error

We have presented four different ways to sample from a population

Each of these involves randomness in the sample-selection process. The error caused by randomness in the sampling process is called sampling error.

Non-sampling error

When sampling from some types of population — especially human populations — problems often arise when conducting one of the above sampling schemes. For example, some sampled people are likely to refuse to participate in your study.

Such difficulties also result in errors and these are called non-sampling errors. Non-sampling errors can be much higher than sampling errors and are much more serious.

It is therefore important to design a survey to minimise the risk of non-sampling errors.