Define systematic and opportunity sampling and state one difference between them.

Systematic sampling refers to a way of choosing participants for a study by taking a list of all participants in a population and choosing every nth number from the list. For example, every 10th or 15th person. Opportunity sampling refers to a way of choosing participants for a study by simply asking people who are nearby, for example, walking down the street, if they would like to take part in the study and so consenting individuals will be chosen as participants. One difference between systematic and opportunity sampling is that systematic sampling tends to have a more representative sample than opportunity sampling which is likely to only use participants who live in the same place, thereby having a similar socio-economic background as well.