AASHTO R 9:05(2018) Standard Practice for Acceptance Sampling Plans for Highway Construction.
8.4. The PWL procedure produces an estimate of the population. Conceptually, as the sample size. n, increases, the estimate of the population improves. The PWL concept is one of small sample sizes and the distribution used is the noncentral t distribution. This distribution converges with the normal distribution when ii approaches infinity. Because PWL is an estimate of a population. it must use an estimate of both the central tendency and the variability.
8.5. SpecifIcation and Acceptance Limits—In the development of an acceptance plan, specification and acceptance limits must be determined, and the acceptable quality level (AQL) and rejectable quality level (RQL) must be selected. The AQL, RQL, specification limits, and acceptance limits are intimately related, and the decisions regarding these are typically made concurrently.
8.6. SpecifIcation Limits—The limiting value(s) placed on a quality characteristic, established preferably by statistical analysis, for evaluating material or construction within the specification requirements. The term can refer to either an individual tipper specification limit (USL) or lower specification limit (LSL), called a “single specification limit”; or to USL and LSL together, called 4bdouble specification limits” (Transportation Research Circular Number E-C037, 2002). As discussed below, specification and acceptance limits are different and serve different purposes.
8.7. Specification limits are based on engineering requirements and are expressed in the same units as those used for the quality characteristic of concern (e.g., percent, millimeters, kilograms per square millimeter, etc.). The acceptance limits are the limiting values placed on a quality measure that will permit acceptance of the lot. In PWL acceptance plans they are expressed in statistical units (PWL).
8.8. Establishing the Specification Limits—Establishing specification limits requires determining what constitutes acceptable (AQL) and unacceptable (RQL) material. The determination of both AQL and RQL are engineering decisions. The AQL decision defines acceptable material or construction, and addresses the product that will provide satisfactory service at an affordable cost when used for the intended purpose. Acceptable material is often determined based on what has performed well in the past. However, if performance data are available, it is preferable to quantify performance.AASHTO R 9 pdf download.