Sample size determination for seed potato certification

Seed buyers desire a conservatively low field certification rate (to better protect their industry). In contrast, seed growers want a relatively higher field certification rate (to reduce their costs in cultural practices which protect against disease). Sampling error requires 1) the grower’s “aimed-for” disease rate to be lower than 2) the agency specified field certification rate, which in turn must be lower than 3) the disease rate which the buyer would find “utterly” unacceptable. Since sample size determines sampling error, it also determines the amount by which the grower must aim to set rate 1) below that of rate 3) to reach reasonable likeliness for his field to be accepted. That is, in a sense, sample size determines the spread between rates 1) and 3). Formula, tables and graphs herein, for given probabilities of error, illustrate the relation between sample size and the required spread in disease rates 1) and 3) where spread is treated as the ratio of rate 1) to rate 3). Such can be useful to certification agencies in making decisions upon sample size requirements for determining field certification.