Selection of a sample that is not a simple random sample is one of the main sources of bias in experiments. Bias can be defined as a systematic tendency to misrepresent the population. So, if the ANN recogniser is intended to be used by all members of the population, you cannot select an unbiased sample of speakers from a sample of people recorded just between 9 a.m. and 5 p.m. This would exclude people who are at work; so, if you do this, the result is a biased sample which is not necessarily representative of the target population.

If you take a sample, how sure can you be that if you measure
variables such as the mean of the sample is close to the mean of
the population? This sort of problem is termed *estimation* and is
considered in the following subsections.

EAGLES SWLG SoftEdition, May 1997. Get the book...