USDA GIPSA Guide: Sampling for the Detection of Biotech Grains - Part 2

Introduction to Sampling Theory

A sample is simply a subset of a lot. Probability theory can describe risk for randomly selected samples. A random sample is one selected in a process in which every possible sample from a lot has an equal chance of being selected.

If every possible sample from a lot could be measured, the average of the measurements would equal the content of the lot. This means that, on average, a random sample produces an unbiased estimate of the measurement of interest.

Measurements on individual samples will deviate from the content in the lot. Probability will not tell what the deviation is on a particular sample, but probability can describe a likely range that the lot content will fall into.

Suppose a random sample of 100 kernels is selected from a lot with five percent biotech kernels. The distribution for this example is given below. A sample from this lot would likely provide an estimate between one and nine percent biotech kernels.

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Increasing the sample size can reduce the range of estimated results. For example, a sample size of 790 kernels would provide an estimate between 3.4 and 6.6 % biotech kernels.

Sampling from Grain Lots

In practice, a pure random sample is not always easy to obtain from a lot. A sampling technique called systematic sampling has been widely used to produce a sample that is a reasonable substitute for a random sample. For example, auditors may use systematic sampling to obtain a sample of files that physically exist in a file cabinet. Suppose 10,000 files are stored in a file cabinet. A sample of 50 files is to be selected for review. Fifty files out of 10,000 files is a rate of one file out of every 200 files. A systematic sampling process starts by selecting a random number between 1 and 200, say 138. Counting through the files, the 138^th, 338^th, 538^th, and so forth, files would be selected for the sample.

In grain inspection, variations of the systematic sampling process are used to select samples. These samples are not random samples by the pure definition, but are approximations from a systematic sampling plan.

Risks can be estimated when random samples are taken. If the sampling procedure is not random, or a close approximation, estimates can be biased. One sampling procedure could be to scoop a sample off the top of a lot using a can. If a lot has been loaded and unloaded many times, the lot may be mixed sufficiently that it is fairly uniform and scooping a sample may be adequate. However, some lots may be the combinations of other lots and the resulting lot can be stratified. Scooping a sample off the top may not be very representative of the lot.

Grain sampling methods prescribed by the Department of Agriculture include methods for sampling moving grain streams and static grain lots. The diverter type (DT) sampler is the most common sampling device for sampling from a grain stream. The DT takes a classic systematic sample. The DT traverses a moving grain stream and, per specific timer settings, diverts a small slice of the grain stream to the inspector. The small slices are combined to obtain the sample for the lot.

A manual means of taking a sample from a grain stream, similar to the diverter type sampler, is the pelican sampler. The pelican sampler is a leather bag on the end of a pole. A person will pass the pelican through a falling grain stream at the end of spout, taking a cut from the grain stream. The pelican is passed through the grain stream frequently. The pelican is emptied between passes through the grain stream.

The Ellis cup is a manual sampling device for sampling from a conveyor belt. A person will frequently dip the Ellis cup into the grain stream. Like the pelican, the Ellis cup is a manual means of taking a sample.

Various probing techniques are used to sample grain from static lots. Depending on the size and shape of the container, multiple probes of the lot will be combined to obtain the sample from the lot. Patterns for probing a lot are prescribed for various types of containers. The individual probes are sufficiently close to effectively sample across any stratification that may exist.

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Example of a Truck Probe Pattern

To obtain the specified test sample size, a sub-sample of the original grain sample must be obtained. Dividers such as the Boerner, cargo, and Gamet have demonstrated the ability to subdivide an origin sample and have the resulting samples conform to distributions expected from a random process.

Detailed instructions for taking samples from grain lots and properly subdividing them for testing are given in the Grain Inspection Handbook - Book 1 and Mechanical Sampling Systems Handbook. Copies can be obtained by contacting the Grain Inspection, Packers and Stockyards Administration of the U.S. Department of Agriculture, ordering through the GIPSA Publications Web site or by viewing/printing from the GIPSA Handbook site page (Requires Adobe Acrobat Reader; this is a 63 page document and may take a minute or so to load).

Eliminating Carry-Over of Biotech Grains

Current testing technology for the detection of biotech grain can be very sensitive, increasing the probability that cross-sample contamination could result in a false positive detection. Furthermore, minor inadvertent commingling of biotech grain kernels with non-biotech grain could result in a positive detection at very low concentrations. Consequently, great care must be taken to ensure the integrity of the grain samples used for testing and to avoid inadvertent commingling during grain handling processes.

Eliminating carry-over of biotech varieties to non-biotech varieties involves understanding and controlling the critical points in the grain handling system. If grain handlers choose to segregate biotech and non-biotech grains, the vehicles, tools, and conveying equipment used in shipment, collection, and transportation of bulk grains throughout the distribution system must either be cleaned before loading non-biotech crops, or dedicated to non-biotech crops. The complexity and cost related to such a process has lead some companies to implement identity preservation systems, rather than segregate non-biotech grain through the traditional marketing system.

Cleanliness of sampling tools is crucial in maintaining the integrity of the system chosen to handle non-biotech crops as well. Manual devices such as pelican samplers or Ellis cups are quite easy to clean, requiring only a visual examination to check no grain or dust remains. Trier probes also require checking and cleaning when moving from sample to sample. Small amounts of grain left in the bottom of a sampling device may result in erroneous results if analyzed for biotech grains. Disassembling and cleaning sampling devices with water or pressurized air may provided added protection against cross-over, but there is insufficient evidence at this time to determine if such measures are necessary. For example, if 10 kernels of biotech corn (about 3 grams) were left in a sampling probe and the probe was used to sample a non-biotech grain lot, those 10 kernels may carry over into the non-biotech sample. If the total sample volume from the probe was 2000g, the resulting percent biotech material in that sublot sample would be 0.15%. This would not be representative of the load being sampled. Similar precautions should be practiced with other types of samplers; any location at which grain or dust may accumulate should be checked and cleaned. A quick visual inspection to ensure no materials have been left behind or caught in the system will avoid carry-over.

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