Closed-form equations to design single sampling plans for isolated lots

In this paper, the problem of designing simple sampling plans by attributes for isolated lots is considered. The customary design procedure is based on numerical tables like those reported in the ISO 2859 norm or on iterative algorithms implemented in a computer software, not suitable for inclusion in a norm. The paper proposes a procedure based on quite simple closed-form equations, yielding the required sample size and the acceptance number for any consistent set of specifications on the operating characteristic, in terms of AQL, RQL, PR CR. The equations are derived using a Gaussian approximation of the hypergeometric distribution. It is shown that the proposed design procedure, besides being very simple, is accurate (i.e. the specification are accurately met) and universal (i.e. the method is valid for a wide range of specifications)