A sampling method is biased when one or more members of the population have an increased chance of being selected compared to the rest of the population. Two examples of sampling methods that produce biased samples are voluntary response sampling and convenience sampling.
Voluntary Response Sampling
When voluntary response sampling is used, the sample consists of members of the population who have volunteered to participate in the research study. This leads to a biased sample because people with greater interested in the topic of the study are much more likely to volunteer.
Convenience sampling occurs when members of the population are selected because they are conveniently accessible to the researcher. An example of convenience sampling would be to stand in a city square and ask passersby to participate in a survey. This results in a biased sample because only the members of the population that pass through the city square at that particular moment have a chance of being selected.