Lot quality assurance sampling

Lot quality assurance sampling (LQAS) is a simple, low-cost random sampling methodology developed in the 1920s to control the quality of output in industrial production processes.

LQAS involves taking a small random sample (a 'lot') and then testing a sample of this lot for quality or the desired characteristic.

It is often stated that 19 'lots' is generally a good number, although the justification for this is unclear. One possibility is that a sample size of 19 provides an acceptable level of error for making management decisions; at least 92% of the time, it identifies whether a coverage benchmark has been reached or whether a lot is substantially below standard. Given this, samples larger than 19 would have practically the same statistical precision as 19. Thus, they would not result in better information, but would cost more to measure.