C. Statistical Analysis
The sample test results for the grade factors, moisture, chemical composition, and physical factors were summarized as the U.S. Harvest Aggregate and also by two groups. The groups, which harvest sorghum in differing time periods, were labeled as Harvest Areas:
- The Early Harvest Area, which consists of areas that typically harvest sorghum from the beginning of July through the end of September; and
- The Late Harvest Area, which consists of areas that typically harvest sorghum from the beginning of September through the end of November or later.
In analyzing the harvest sample test results, the Council followed standard statistical techniques employed for proportionate stratified sampling, including weighted averages and standard deviations. In addition to the weighted averages and standard deviations for the U.S. Harvest Aggregate, weighted averages and standard deviations were calculated for the Harvest Areas. First, each sampled ASD was categorized by Harvest Area, based on historical USDA state-level harvest progress data, with each ASD exclusively belonging to one Harvest Area. Second, each ASD was weighted by its estimated proportion of foreign exports. The Harvest Area and U.S. Harvest Aggregate statistics were calculated using these weights.
The Relative ME was calculated for each of the quality factors for the U.S. Harvest Aggregate and for each of the Harvest Areas. The Relative ME for the quality factor results was less than ± 10%, except for total damage for the U.S. Harvest Aggregate and Early Harvest Area, and for foreign material and BNFM for the Early Harvest Area. The Relative ME for total damage, foreign material, and BNFM are shown in the table to the right.
While the level of precision for these quality factors is lower than desired, these levels of Relative ME do not invalidate the estimates. Footnotes in the summary tables for “Grade Factors and Moisture” indicate the attributes for which the Relative ME exceeds ± 10%.