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Apart from the sampling errors related to the sample preparation protocol in a laboratory to determine the ash and sulfur content of the coal, the delimitation/extraction errors (recovery, among other geological factors) and the “extension” error must be considered. As the collection time interval increases, the extension error also increases, so that even automatic sampling has an error that is often completely ignored. The extension error was determined via a variographic test, which was specifically designed for this study. This test was proposed to be carried out at the mining company to check the uncertainties (bias and reproducibility errors), regarding the sampling at the client. In the latter, sampling is done in piles with manual samplers. At the mining company, there are automatic collectors, but the sample interval is not considered when calculating the total error. To redeem reconciliation problems between the mining company and the client, as well as to avoid contractual fines, it was decided to compare the results of the variographic analysis carried out in the mining company with those obtained by the client. These showed considerable bias and extension errors according to the collection time intervals in the mining company, especially regarding sulfur. In shorter time intervals, 30 minutes, extension errors have resulted in a range of [0.18% to 1.52%] and [29% to 59%] for sulfur and ashes respectively (CI = 95%). For longer intervals, there would be a doubt about accepting or not the lot: maximum limits are 1.5% for sulfur and 55% for ashes. Also, as extension errors are elevated and possibly higher for “true mass flows,” which exceed 2,000 t/h, it was recommended to adapt the automatic samplers of the mining company at shorter intervals than it was used to do (2/2 hours). Then, it will improve the processes control, including blending, avoiding reconciliation problems, and subsequent contractual fines. The client was alerted to the problem of bias which occurs when the coal is sampled manually in piles. For both, it is important to know the intrinsic variance of the material/lot and the errors related to the sampling.

The most ignored error in the history of mining sampling – extension variance: a case study applied to coal