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% Disease Estimation and Sample Size Tools

Currently, the disease estimates reported by certification agencies provide a measure (%) of diseased plants/tubers in the sample that is observed or analyzed. For example, the number of diseased plants in the 400 tuber sample sent to the post-harvest test or some number of plants observed during the walk through of the field during the summer. This number is not an estimate of the disease in the entire seed lot nor does it take into consideration the variation associated with tuber or plant sampling. That is, if you were to sample the same seed lot 5 times you would find the 5 numbers don’t necessarily match. This variation occurs because the distribution of the virus is an unknown, there is always a level of uncertainty that any given sample will be truly representative of the entire seed lot.

The Excel tool provided in the file below allows the user to determine a more precise estimate of % disease in the entire seed lot and provides a range, or confidence interval, which takes into account the uncertainty associated with sampling. The confidence interval can be adjusted to reflect a grower’s risk tolerance, e.g. a grower would like to be 95% certain the estimate and range are precise. The lower the risk tolerance, the wider confidence interval and vice versa.

The tool can be used to calculate estimates of % disease after testing or it can be used in advance to determine the sample size necessary to obtain a specified target disease level at a growers specified risk tolerance. Larger samples will always allow the detection of disease at lower levels and with greater accuracy, but are not always feasible.

The tool is a modification of the Seedcalc8 tool that can be found on the International Seed Trade Association (ISTA) website ISTA Statistical Tools for Seed Testing. The tool was developed by Kirk Remund (US), Robert Simpson (US), Jean-Louis Laffont (FR), Deanne Wright (US), Sylvain Gregoire (FR) to design seed testing plans for the presence of GM traits in conventional seed lots. The tool is freely available to the public, but please review the worksheet “Information and Disclaimer” prior to using the tool.

We have removed many of the worksheets in the original Seedcalc8 file and retained the “Information & Disclaimer” worksheet plus the two worksheets most relevant to disease testing in seed potato lots. Modifications of the text were made as appropriate, however, the original text has been retained on the “Information & Disclaimer” worksheet.

There are 3 worksheets in the Seedcalc8 for Potato Disease Testing Excel file as noted, below. These worksheets are password protected to prevent inadvertent deletions or other changes.

  • Introduction & Disclaimer – Please read the warranty and introduction sections on this worksheet.
  • Individual Tuber-Plant Testing
  • Composite Tuber-Plant Testing

Important Assumptions for Using These Tools

The confidence intervals are an F-distribution approximation.  The validity of these calculations are based on the following assumptions:

1) The number of tubers/plants sampled should not exceed 10% of total number in the seed lot.
2) A random sample of tubers/plants are taken from the seed lot for testing.
3) The diseased tubers/plants are evenly distributed across the seed lot.

Important Note:  When entering percentages less than 1% in Excel, precede the decimal point with a zero.  For example, to enter .45 % type 0.45.  Failure to include the leading zero will result in an incorrect value (typing only .45 will be converted to 45.0 %).




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