Survey and Statistical Analysis Methods
Sample Design and Sampling
We applied a proportionate stratified, random sampling technique to ensure a sound statistical sampling of the U.S. corn crop at the first stage of the marketing channel. Three key characteristics define the sampling technique: the stratification of the population to be sampled, the sampling proportion per stratum, and the random sample selection procedure.
Stratified sampling involves dividing the survey population of interest into distinct, non-overlapping subpopulations called strata. For this study, the survey population was corn produced in areas likely to export corn to foreign markets. The U.S. Department of Agriculture (USDA) divides each state into several Agricultural Statistical Districts (ASDs) and estimates corn production for each ASD. The USDA corn production data, accompanied by foreign export estimates, were used to define the survey population in twelve key corn producing states representing 98% of the 2010 U.S. corn exports (USDA). From those data, we calculated each ASD’s proportion of the total production and foreign exports to determine the sampling proportion and ultimately, the number of corn samples to be collected from each ASD. The number of samples collected for the Harvest Report differed from ASD to ASD because of their different shares of estimated production and foreign export levels.
The random selection process was implemented by soliciting country grain elevators in the twelve states by mail, fax, e-mail and phone. Postage-paid sample kits were mailed to elevators agreeing to provide the 2,050 to 2,250 gram corn samples requested. Samples were collected from the elevators when at least 30% of the corn in their area had been harvested. The 30% harvest threshold was established to avoid receiving old crop corn samples as farmers cleaned out their bins for the current crop or new crop harvested earlier than normal for reasons such as elevator premium incentives. The individual samples were pulled from inbound farm-originated trucks when the trucks underwent the elevators’ normal testing procedures. In addition, elevators were requested to submit only samples of corn with up to 22% moisture. This criterion was used to prevent sample deterioration during shipping. The number of samples each elevator provided for the survey depended on the targeted number of samples needed from the ASD along with the number of elevators willing to provide samples. A maximum of four samples from each physical location was collected.