VI. SURVEY AND STATISTICAL ANALYSIS METHODS
The key points for the survey design and sampling and statistical analysis for this Harvest Report 2013/14 are as follows:
- Following the methodology developed for the Harvest Reports 2011/12 and 2012/13, the Council proportionately stratified the samples according to Agricultural Statistical Districts (ASDs) across 12 key corn producing states representing 99% of U.S. corn exports.
- A total of 600 samples collected from the 12 states was targeted to achieve a maximum ±10% relative margin of error (Relative ME) at the 95% confidence level.
- A total of 610 unblended corn samples pulled from inbound farm-originated trucks was received from local elevators from Sept. 9 through Dec. 4, 2013, 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 179 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 three attributes – total damage, 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 2013 and 2012 and the 2013 and 2011 quality factor averages.