Statistical Analysis
The sample test results for the grade factors, 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. We labeled these “Export Catchment Areas” (ECAs) as follows: the Gulf ECA consisting of areas that typically export corn through the U.S. Gulf ports, the Pacific Northwest (PNW) ECA comprising of areas that export corn through Pacific Northwest and California ports, and the Southern Rail ECA that includes areas generally exporting corn by rail to Mexico.
In analyzing the sample test results, we followed the standard statistical techniques employed for proportionate stratified sampling including weighted averages and standard deviations. (In some instances, the ASDs were over-sampled, and in those cases, the statistics were adjusted to account for the oversampling.) 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. These estimations were based on industry input and evaluation of studies of grain flow in the U.S.