Before an organisation conducts primary research it needs to pick a list of people/organisations to interview. You can not interview the whole population; that would be expensive and probably impossible. Instead if you want to conduct research which involves interviewing people, you will need to select a group of people to interview. There are a number of ways to select the sample of people to be interviewed; this article covers probability sampling and non-probability sampling.
The diagram below summarises probability and non-probabaility sampling methods.
Probability Sampling Methods
Probability sampling methods are said to be mathematical ways to select the group of people that will be interviewed. They are known as probability sampling methods because each person in the group from which the sample will be selected has a "known" chance (probability) of being selected. There are three probability sampling methods: Simple random samples, systematic samples and multi-stage samples.
Simple Random Samples
Simple random sampling involves making a list of the people from which the sample of respondents (people that will be interviewed) is selected. For example a group of 100 are listed and a group of 20 may be selected from this list at random. The selection may be done by computer or by hand.
Just like simple random sampling, systematic sampling involves drawing up a list of potential respondents. The next stage is to decide what system will be used to select the respondents from the list. For example every 5th person on the list will be selected as a respondent so that would be the 5th, 10th, 15th, 20th person and so on. Similarly if the interviewers implement a 10th person system, they would choose the 10th, 20th, 30th, 40th, 50th person and so on.
With this sampling process the respondents are chosen through a process of defined stages. For example residents within Islington (London) may have been chosen for a survey through the following process:
Throughout the UK the south east may have been selected at random, ( stage 1), within the South East London is selected again at random (stage 2), Islington is selected as the borough (stage 3), then polling districts from Islington (stage 4) and then individuals from the electoral register (stage 5). As demonstrated five stages were gone through before the final selection of respondents were selected from the electoral register.