| Popis: |
A family of power transformations was examined to improve effectiveness of matitis diagnosis and sensitivity of hypothesis tests. The family of transformations considered included square root, logarithmic, reciprocal and reciprocal of square root. N- acetyl -β- d - glucosaminidase activity was assayed on foremilk samples collected as composites of all quarters from each of 488 cows in ten commercial herds. Herds were sampled three times at monthly intervals. The frequency distribution of the observed assay results was extremely right skewed. Although the optimal transformation was reciprocal of the square root, the logarithmic transformation was only slightly inferior and is recommended for practical application. After transformation by logarithm, skewness was greatly reduced (1.35 vs. 0.13), coefficient of determination increased (0.81 vs. 0.86), F value for the full model increased (3.36 vs. 4.80), and the contrast in standard deviation units between samples with major pathogen and no infection was increased by 65%. Choice of base for logarithmic transformation is immaterial because the frequency distributions of standard unit deviations from means are invariant to the base. |
