Stringent sampling inspection plans using upper envelopes for cell counts

A new type of stringent sampling inspection plan is proposed for cell counts generally in line with a Poisson or a mixed-Poisson type of distribution. The proposed plan is based on setting an upper envelope estimated from the sample counts which should remain below the given microbiological limit m. We used Monte-Carlo simulation methodology to evaluate the proposed plans and showed that the lot acceptance criterion using the upper envelope improves consumer protection in general. Both two and three class attribute plans are investigated for establishing the upper envelope. Our study reveals that the proposed plans can offer better consumer protection and economy when compared to both two and three class attribute plans. It is also demonstrated that a slightly stringent plan can be found in place of lognormal type variables plans. The limitations of the proposed new plans, and a discussion on alternative stringent rules for batch acceptance are also provided. The new plan was also validated using the APC cell counts observed for 2000 milk powder batches. Our conclusion is that the proposed stringent plans can improve protection to the consumer, particularly for exported food products.