WITHIN-MILL VARIATION IN THE MEANS AND VARIANCES OF MOE AND MOR OF MILL-RUN LUMBER OVER TIME
Keywords:mill run, full lumber population, modulus of rupture, modulus of elasticity, mean, variance, seasonal difference, mixed normal distribution
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 MOE and modulus of rupture (MOR) in mill-run lumber populations at the same mill over time. The objective of this study was to investigate if statistically significant differences between the means and variances of MOE and MOR 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 2x4 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 MOE and/or mean MOR of the summer and winter samples from Mills 2 and 4, significant differences in mean MOE and/or MOR were found between the summer and winter samples from Mills 1 and 3. In addition, a Levene’s test on the MOR of Mill 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 grade material than the summer sample, there were pronounced strength differences between the summer and winter samples both around the median and at the lowest (near-minimum) percentiles within each grade. This reinforces the notion that changes in mill-run MOR distributions 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.
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