Pierre Gy's theory for the sampling of particulate materials is widely applied and taught. A crucial part of Gy's theory deals with the estimation, prediction and minimization of the variance of the Fundamental Sampling Error using a formula that is known as "Gy's formula". Experimental evidence, however, supports the conclusion that Gy's formula is inaccurate. (C) 2010 Elsevier Ltd. All rights reserved.