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Source: SGS Seed and Crop Services May Newsletter
by Kalyn Brix-Davis, Genetic Services Director, Branch Manager

With seed testing that may require individual seed analysis for specific trait or events and/or a shortage of seed available for testing in situations such as elite inbred seed, sequential testing is a tool that can manage risk, short seed stocks and expense of testing.  Sequential testing is a process in which a testing plan is developed based on statistics on the amount of risk associated with testing a minimum amount of seed at the initial testing period.  The level of rejection is higher than normal to allow for a lower amount to be tested at the initial period, while a secondary tier of testing is established when this level of rejection is reached.  In the overall scheme, the amount of primary and secondary testing required is less than what would be required with the lower level of rejection required with testing more seeds during the initial testing period. 

SeedCalc7 was developed by seed quality statistician to assist in calculation of purity/impurity levels in seed lots with the ability to set the confidence level and determine the amount of risk that is associated with different sampling/testing schemes.  This Microsoft Excel program is available at the International Seed Testing Association Statistics committee website (http://seedtest.org/en/content---1--1143.html). 

In the table below are various number of seeds that are tested in genetic or trait purity testing (growouts, electrophoresis, insecticide trait) with different number of deviants for the testing and the calculated purity level and the upper bound limit at a 95% level.   In our example, a 95% confidence level that means that the purity level will be below the upper bound limit 95% of the time.

For example, if 50 seed electrophoresis test resulted with 0 offtypes or selfs, the calculated purity level would be 100% with a upper bound level of 5.82% impurity.  With 1 deviant in 50 seeds, the purity level is 98% with an upper bound level of 9.14% impurity.  If an additional 50 seeds are tested and 0 deviants (total tested = 100, 1 deviant), the purity level is 99% with an upper bound level of 4.66% impurity.

By using sequential sampling, a testing plan can be devised to maximize information and costs while testing genetic purity to a confidence level and quality standard that your company is comfortable in providing to your customer.  If you would like additional information, please contact Kalyn at kalyn.brixdavis@sgs.com.

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