Critical sample size
Sample size depends on the tolerance for the character to be examined.
By tolerance is meant: the proportion of infected plants and tubers, respectively, which is acceptable for the purpose concerned.
A principal fist rule is that the sample size should be so big that the number of infected tubers to be detected should be at least 4.
This number 4 is based on the chance that in 95% of the lots with the expected level of contamination one or more infected tubers are detected.
This means that in case there is a tolerance of 2% (is 2 tubers per 100, or 4 tubers per 200), sample size should be 200. In case the tolerance is 0.1% (which is 1 tuber per 1000 or 4 tubers per 4000), 4000 tubers should be the minimum sample size.
Field inspection versus laboratory testing
It is obvious that in many cases, concerning seed certification schemes, it is difficult if not impossible for economic reasons to comply with the large sample sizes required.
It also indicates that laboratory testing of random samples of limited size can never replace the field inspection procedures, as in the field a single aberrant plant per 1000 is easily detected by visual inspection.
Compromise
With respect to sample size you have to compromise between danger/importance of the disease and the feasibility of the testing procedure.
However where concerning quarantine diseases such compromises are not acceptable.