The main problem in treating data statistically is how to ensure the reliability of information about the population obtained from a sample. For this, the sample must be representative. The main requirement to achieve this is to take a simple random sample . A sample is a simple random sample if every member of the population has the same chance of being selected as every other member. Thus, if in setting up the ANN recogniser , speakers from the lab are used to obtain training data , the sample would not be simple random: It is unlikely that the members of the lab give all social strata, or females, or people outside working-age, a fair chance of being selected.