A tire was measured to determine its average groove depth by each of several operators in each of seven laboratories in the morning and afternoon for two consecutive days. The results show that the operators repeat their measurements within acceptable variation. On averages of six measurements by a single operator, a difference of 0.002 in. (0.05 mm) was significant. The variability of operators' averages around their laboratory average was the same for all laboratories except one. However, averages among laboratories were different, which is attributed to small differences in the measuring practice. The precision of wear rate estimates based on the difference of average groove depths before and after a period of wear is estimated and ways in which the precision of estimates of wear rates can be optimized are suggested.

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