Statistical Analysis
The sample test results for the grade factors, moisture, chemical composition, and physical factors were summarized as the U.S. Aggregate and also by three composite groups that supply corn to each of three major export channels, labeled Export Catchment Areas (ECAs), as follows:
The Gulf ECA consists of areas that typically export corn through the U.S. Gulf ports;
The Pacific Northwest (PNW) ECA includes areas that export corn through Pacific Northwest and California ports; and
The Southern Rail ECA comprises areas generally exporting corn to Mexico by rail from inland subterminals.
In analyzing the sample test results, the Council followed the 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. Aggregate, weighted averages and standard deviations were estimated for the composite ECAs. The geographic areas from which exports flow to each of these ECAs overlap due to available transportation modes. Therefore, composite statistics for each ECA were calculated based on estimated proportions of grain flowing to each ECA. As a result, corn samples could be reported in more than one ECA. These estimations were based on industry input, export data, and evaluation of studies of grain flow in the United States.
New to this 2016/2017 Harvest Report is a simple average of the quality factors’ averages and standard deviations of the previous five Harvest Reports (2011/2012, 2012/2013, 2013/2014, 2014/2015, and 2015/2016). These simple averages are calculated for the U.S. Aggregate and each of the three ECAs and are referred to as “5YA” in the text and summary tables of the report.
The Relative ME was calculated for each of the quality factors for the U.S. Aggregate and each of the ECAs. The Relative ME was less than ±10% for all the quality attributes, except for stress cracks and SCI, for the U.S. Aggregate and the Gulf, Pacific Northwest, and Southern Rail ECAs. The Relative ME for stress cracks and SCI are shown in the table below.
While the lower level of precision for these quality factors is less than desired, these levels of Relative ME do not invalidate the estimates. Footnotes in the summary tables for “Grade Factors and Moisture” and “Physical Factors” indicate the attributes for which the Relative ME exceeds ±10%.
References in the “Quality Test Results” section to statistical and/or significant differences between results in the 2015/2016 Harvest Report and the 2016/2017 Harvest Report, and in the 2014/2015 Harvest Report and the 2016/2017 Harvest Report, were validated by two-tailed t-tests at the 95% confidence level.