Within-mill variation in the means and variances of MOE and mor of mill-run lumber over time

The literature related to the phenomenon of pseudo-truncation has emphasized that the mechanical property distributions of graded lumber subpopulations are determined by the mechanical property distributions of the mill-run (or full) lumber populations from which the subpopulations are formed. Whereas previous studies have shown that the means and variances of mechanical properties in the same visual grade of lumber can vary from mill to mill, there have been no studies on the stability of the means and variances of modulus of elasticity (MOE) and modulus of rupture (MOR)in mill-run lumber populations at the samemill over time. The objective of this study was to investigate if statistically significant differences between the means and variances ofMOE andMOR in mill-run lumber populations at the same mill could be observed across samples taken several months apart. Two mill-run samples of 200 pieces of rough, dry 2×4 southern pine lumber were taken from each of four Mississippi sawmills: one in the summer and one in the winter. For each mill, the summer and winter means and variances of flexural MOR and MOE were compared. Whereas no significant differences were found between the mean MOR or mean MOE of the summer andwinter samples from Mills 2 and 4, significant differences in meanMOEand/orMORwere found between the summer and winter samples fromMills 1 and 3. In addition, a Levene's teston theMORofMill 1 showed significant differences in the variance between the summer and winter samples. Further analysis revealed that in addition to the fact that the winter mill-run sample from Mill 3 was made up of a larger percentage of lower gradematerial than the summer sample, there were pronounced strength differences between the summer and winter samples both around themedian and at the lowest (near-minimum) percentiles within each grade. This reinforces the notion that changes inmill-runMORdistributions over time can have an important effect on the overall strength of a given mill's visual grades over time. A theory of mixed distributions could account for these differences.