Article - "The Effect of Lot Size"

The Effect of Lot Size

Mil-Std-105E indexes sampling plans by AQL, Levels of Inspection, and lot size. It is a common misconception that 105E includes lot size because "larger lots require more samples to obtain the same level of protection." In actuality 105E takes more samples from larger lots in order to get better protection. Figure 1 shows the OC curves of several of the 1.0% AQL sampling plans. Increasing lot size increases the sample size letter code which steepens the OC curve resulting in better protection.

Figure 1: AQL=1% OC Curves

Figure 1: AQL=1% OC Curves

Better protection for larger lots can be justified by the fact that for larger lots the costs of rejecting good lots and the costs of accepting bad lots are higher. Since the consequences of making wrong decisions are higher, it is logical to take more samples to lower the risk of making wrong decisions.

While this justification has merit when considering a single product, 105E is used to inspect a variety of products. Should more samples be selected from a large lot of pencils or from a small lot of pace makers? To overcome this objection, different levels of inspection are provided. 105E states that these inspection levels are to be selected based on the "discrimination" required.

The "discrimination" or protection provided by a sampling plan depends primarily on the number of units inspected and the acceptance number. Lot size has only a minor effect limited to the case when 10% or more of the lot is inspected. As a result, the single sampling plan n=13 and a=0 provides the same protection regardless of whether the lot size is 50, 200, or 200,000. Figure 2 shows OC curves for these different lot sizes. OC curves based on lot size, are called Type-A OC curves (hypergeometric distribution). They are closely approximated by the Type-B OC curve which assumes an infinite lot size (binomial distribution). The Type-B OC Curve represents worse case. It has the greatest chance of both accepting bad lots and rejecting good lots.

Figure 2: OC Curves of n=13 and a=0

Figure 2: OC Curves of n=13, a=0

Since Type-B OC curves represent worse case, sampling plans selected based on Type-B OC curves can be used to inspect any lot regardless of size. When selecting statistically valid sampling plans, it is not necessary to use different sampling plans for different lot sizes. A better strategy is to select one sampling plan based on the protection it provides, i.e., its OC curve. The OC curves of the 105E plans are given in Table X of 105E. Tables of sampling plans indexed by their OC curves are given in my book.

Appeared in FDC Control, Food Drug & Cosmetic Division ASQC, No. 103, September 1994, p. 6