With simple random sampling, a sample is chosen in such a way that each possible sample has an equal chance of being selected from the population. When a researcher for example wants to select a sample of 100 participants from a population of 5000 participants and each combination of 100 participants has an equal chance of being selected as sample, it is a simple random sample. To select such a sample, the researcher should use a sampling frame. That is a list for the whole population from which the sample will be drawn. Participants are selected randomly from this list. A disadvantage of the simple random sampling is that it requires to know beforehand how many participants there are in the population, and how many are required for the sampling frame. In some situations, forming a sampling frame is impossible.