Sampling error is caused by observing a sample (some of the population) instead of the full population. It is inherent uncertainty, arising from running a survey — rather than a census. We are usually interested in how much the survey estimate could plausibly differ from the population.
There are three notable misunderstandings when people talk about surveys, such as political opinion polls: the sampling error of small proportions, the treatment of sub-samples, and total survey error. This article examines these three misunderstandings.