B. Survey Design and Sampling
1. Survey Design
For this 2015/2016 Harvest Survey, the target population was commodity sorghum from the nine key U.S. sorghum-producing states representing more than 98% of U.S. sorghum exports. A proportionate, stratified, random sampling technique was applied to ensure a sound statistical sampling of the U.S. sorghum crop at the first stage of the marketing channel. Three key characteristics define the sampling technique: the stratifcation of the population to be sampled, the sampling proportion per stratum, and the random sample selection procedure.
Stratifcation involves dividing the survey population of interest into distinct, non-overlapping subpopulations called strata. For this study, the survey population was sorghum produced in areas likely to export sorghum to foreign markets. The U.S. Department of Agriculture (USDA) divides each state into several ASDs and estimates sorghum production for each ASD. The USDA sorghum production data, accompanied by USDA sorghum consumption data and foreign export estimates, were used to define the survey population in nine key sorghum producing states representing more than 98% of U.S. sorghum exports. The ASDs were the subpopulations or strata used for this sorghum quality survey. From those data, the Council calculated each ASD’s proportion of the total U.S. foreign exports to determine the sampling proportion (the percent of total harvest samples per ASD) and ultimately, the number of sorghum harvest samples to be collected from each ASD. The number of samples collected for the 2015/2016 Harvest Survey differed from ASD to ASD because of the different shares of estimated foreign export levels.
The number of harvest samples collected was established so the Council could estimate the true averages of the various quality factors with a specifc level of precision. The level of precision chosen for the 2015/2016 Harvest Survey was a Relative ME no greater than ± 10%, estimated with a 95% level of confidence. A Relative ME of ± 10% is a reasonable target for biological data such as these sorghum quality factors.
To determine the number of harvest samples for the targeted Relative ME, ideally the population variance (i.e., the variability of the quality factor in the sorghum at harvest) for each of the quality factors should be used. The more variation among the levels or values of a quality factor, the more harvest samples required to estimate the true mean within a given confidence level. In addition, the variances of the quality factors typically differ from one another. As a result, different sample sizes for each of the quality factors would be needed for the same level of precision.
When population variances are not known, variance estimates from similar data sets are used. Although a reliable source of chemical composition and physical factor data was not available, variances and Relative MEs for the grade factors were calculated using USDA’s Grain Inspection, Packers and Stockyards Administration (GIPSA) Farm Gate Studies from 2007 through 2010, and were used as proxies. The variances and the estimated number of harvest samples required for the Relative ME of ± 10% for the grade factors were ultimately determined by examining these studies.
Based on these data, a total sample size of 200 would allow the Council to estimate the true averages of the grade factor characteristics with the desired level of precision for the U.S. Harvest Aggregate, with the exception of total damage.
The same approach of proportionate stratified sampling was used for the mycotoxin testing of the sorghum harvest samples as for the testing of the grade, moisture, chemical, and physical characteristics. In addition to using the same sampling approach, the same level of precision of a Relative ME of ± 10%, estimated with a 95% level of confidence, was desired. Testing at least 50 harvest samples (25% of the 200 targeted harvest samples) would ensure with 95% confidence that the percent of tested harvest samples with aflatoxin results below the U.S. Food and Drug Administration (FDA) action level of 20 parts per billion (ppb) would have a Relative ME of less than or equal to ± 10%. It was also estimated that the percent of tested harvest samples with DON results below the FDA advisory level of 5 parts per million (ppm) would have a Relative ME of less than or equal to ± 10%, estimated at a 95% level of confidence. The proportionate stratified sampling approach also required testing at least one sample from each ASD in the sampling area. To meet the sampling criteria of testing 25% of the total number of targeted harvest samples (200) and at least one sample from each ASD, the targeted number of harvest samples to test for mycotoxins was 58 samples.
The random selection process was implemented by soliciting local grain elevators in the nine states by email and phone. Postage-paid sample kits were mailed to elevators agreeing to provide the 2500-gram sorghum samples requested. Samples were collected from the elevators when at least 30% of the sorghum in their area had been harvested. The 30% harvest threshold was established to avoid receiving old-crop sorghum 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. 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 seven samples from each physical location was collected, but nearly 90% of the participating elevators submitted four or fewer samples. A total of 207 unblended sorghum samples pulled from inbound farm-originated trucks were received from local elevators from August 28, 2015 through January 5, 2016, and tested.