A. Overview

The key points for the survey design and sampling and statistical analysis for this 2016/2017 Harvest Report are as follows:

  • Following the methodology developed for the previous five Harvest Reports, the samples were proportionately stratified according to Agricultural Statistical Districts (ASDs) across 12 key corn-producing states representing 93.1% of U.S. corn exports.
  • A total of 617 samples collected from the 12 states were targeted to achieve a maximum ±10% relative margin of error (Relative ME) at the 95% confidence level.
  • A total of 624 unblended corn samples pulled from inbound farm-originated trucks were received from local elevators from September 8 through November 28, 2016, and tested.
  • A proportionate stratified sampling technique was used for the mycotoxin testing across the ASDs in the 12 states surveyed for the other quality factors. This sampling resulted in 177 samples being tested for aflatoxins and DON.
  • Weighted averages and standard deviations following standard statistical techniques for proportionate stratified sampling were calculated for the U.S. Aggregate and the three Export Catchment Areas (ECAs).
  • To evaluate the statistical validity of the samples, the Relative ME was calculated for each of the quality attributes at the U.S. Aggregate and the three ECA levels. The Relative ME for the quality factor results was less than ±10% except for two attributes – stress cracks and stress crack index (SCI). While the lower level of precision for these quality factors is less than desired, these levels of Relative ME do not invalidate the estimates.
  • Two-tailed t-tests at the 95% confidence level were calculated to measure statistical differences between the 2016 and 2015 and the 2016 and 2014 quality factor averages.