During sampling of particulate materials, correct sampling is defined in Gy's sampling theory

Gy's sampling theory

Gy's sampling theory is a theory about the sampling of materials, developed by Pierre Gy from the 1950s to beginning 2000s in articles and books including:* Sampling nomogram* Sampling of particulate materials; theory and practice...

 as a sampling scenario in which all particles in a population have the same probability of ending up in the sample .

The concentration of the property of interest in a sample can be a biased

Bias (statistics)

A statistic is biased if it is calculated in such a way that it is systematically different from the population parameter of interest. The following lists some types of, or aspects of, bias which should not be considered mutually exclusive:...

estimate, for the concentration of the property of interest in the population from which the sample is drawn. Although generally non-zero, for correct sampling this bias is thought to be negligible .