Precision is expressed as either the standard deviation (SD) or relative standard deviation (RSD) of the method. For a microbiological enumeration method values within 0.5 log are considered precise. To compare the precision of one method relative to another, a suspension should be prepared at the upper end of the test capability, which is then serially diluted down to the lower end of the range. Between 2 to 5 of the dilutions should be compared with at least 10 replicates of each being performed. An SD or RSD in the region of 10 to 15% is considered acceptable. The variance of each method can be statistically compared using the F test enabling any significant difference between the precision of the two methods to be demonstrated.
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