CASE 43
COAL TESTING Coal is a heterogeneous bulk commodity, typically sold under contracted terms that include bonuses and penalties associated with various quality parameters and impurity levels of the coal. In many cases, predicting and controlling the quality of the coal accurately can mean the difference between economic success and failure for a coalmine.
The heating value is a quality of particular interest. It is the basis of payment in most coal supply contracts. In the United States, the heating value of coal is measured in British thermal units per pound (BTU/lb), by using a national standard test procedure (ASTM D-2015). ASTM standards also indicate that repeated measurements should give values within 100 BTU/lb.
Normally, the only way to determine heating value is to send a carefully prepared sample off to a laboratory and wait for the results. This can lead to disputes, if the customer and the supplier don’t agree on which lab to use or if the results from different laboratories give drastically different results.
In an effort to determine the extent of any intra- or inter-laboratory variation in heating value measurements, one coal company prepared 50 samples of coal from a common area (so that results were expected to be very close). They shipped the samples in two batches to five reputable laboratories. Each batch consisted of five samples sent to each lab; the two batches were sent at different times. The samples were drawn randomly, packed in water (to minimize oxidation effects) and sent in sealed plastic containers. The labs were not aware that an experiment was being run, and the coal company hoped that the results would thus reflect the labs’ normal operating procedures.
DATA SET
The labs made several measurements, but only those for heating value are given below. What do you think the coal company should conclude about the performance of the laboratories on its list, at least as far as heating value measurements are concerned?
The data are available in file COALLAB. The variables are: LABNO, the laboratory number; BATCH, the batch number (0 or 1); and BTU, the BTUs per pound as re- ported by the laboratory.