A simplified approach to the demonstration of Gy’s Theory of Sampling (TOS) sampling variance formula is proposed, with the added advantage that it clarifies the real assumptions that are necessary to lead the demonstration to its end. In the process, the introduction of the liberation factor in TOS is also clarified and, for the first time, a rigorous definition of that factor is offered, which naturally lead to its modelling in past years using geostatistical concepts. A generalised from for the mineralogical factor c is also proposed in an appendix.