Abstract
At Cliffs Natural Resources Ltd (CNR) crushing and shipping sample stations, precisions are estimated for the sampling, preparation and measurement stages of iron ore sampling in accordance with ISO 3085 (2002). In this standard, gross duplicate samples are prepared at each of the three stages, yielding eight measurement assays. Appropriate calculations of the differences between assay pairs are then made to estimate precision of each stage of sampling. The international standard for checking precision in iron ore sampling, ISO 3085 (2002), prescribes a method for identifying outliers that is ambiguous and possibly inappropriate. The procedure also makes an unnecessary implicit assumption, that the differences between assay pairs are normally distributed. The Anderson–Darling statistic is used to demonstrate that in some cases the assay pair differences have distributions significantly different from normal. When applied at CNR as specified, the standard leads to precision estimates well below the expected values, thus overestimating the sampling method's capability and limiting the opportunity for real process improvement. This paper suggests an improved method of estimating sampling, preparation and measurement precision. A bootstrap procedure is used to estimate confidence limits for the calculated precision estimates. The proposed method would be suitable for testing blast hole precision. The discussion is supported by extensive simulation modelling of realistic data.
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