Bias in manual sampling of rock particles

This paper examines the hypothesis that the manual selection of rocks for inspection, testing or analysis is invariably biased towards the heavier (larger) particles in the population being sampled. If the property of interest, such as assay or breakage potential, is size-related then such a bias would lead to systematic errors in the estimation of this property. To test the hypothesis, human volunteers were asked to select a sample of 10 rocks from a tray of 100 rocks of known weights, with and without a blindfold, in duplicate. This was repeated for a number of different rock size ranges in the range −50 + 19 mm. A statistical analysis of the results confirms the hypothesis that in almost all cases the samples were of larger weight than that expected from the known weight of the population of rocks. The magnitude of the bias depended on conditions but was highest for the widest size range. It is also shown that the volunteers produced different results to each other. The blindfold reduced the bias in the narrow size ranges but increased it for the wide size range. These effects are likely to be less important for populations of narrow size range, but where a truly unbiased sample is required strategies are proposed using randomisation processes. Relying on unmoderated human selection will lead to samples which overestimate the weight of the population.