Standard test method measurements are subject to inherent random error, systematic error of the procedure (bias), and systematic error introduced by an individual laboratory's procedures [10]. Test results therefore need to be judged considering the inherent variability of the method. Identifying the sources of these errors in the development of a standard test may provide an opportunity to modify the procedures to reduce their influences. The availability of a reference material provides a means to assess the contributions of both random and systematic error through round-robin testing. ASTM provides guidance in conducting a round-robin with standard practices on conducting inter-laboratory studies [11] and preparation and use of precision and bias statements [12, 13].
The precision of a measurement is an assessment of the variability one may expect when the test method is used by one or more reasonably competent laboratories; the ASTM definition being "the closeness of agreement among test results obtained under prescribed conditions" [13]. In addition, two concepts are used in expressing precision: repeatability and reproducibility. Four factors contributing to the test variability according to [11] are: 1) the operator, 2) the equipment, 3) equipment calibration, and 4) the testing environment. Within-laboratory variability (repeatability sr) excludes these four factors while between-laboratory variability (reproducibility sR) includes these four factors. Both measures represent standard deviations of replicate analyses. Bias is the difference between the test result and the true value and will only be briefly discussed here as calculations have not yet been completed.
From the sr and sR, limits on the difference between two test results may be calculated, which are designated 95 % repeatability (r) and reproducibility (R). This is accomplished by multiplying the appropriate standard deviation by the factor 1.96*√2. Methods for estimating r and R and the standard deviations upon which they are based are given in ASTM E 177 [13]. The provisional estimates given here were pooled for each phase from results of the four cements using the standard formula for pooling variances and standard deviations [14].