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Percent Within Limits
There are many ways to measure the quality of pavement material. Most State highway agencies accept pavement material from a contractor and pay that contractor through a system of incentives and disincentives. The Federal Highway Administration (FHWA) is recommending one particular method as its "quality measure of choice." It's called Percent Within Limits or PWL.
The following definition is based on the TRB Electronic Circular E-C074: Glossary of Highway Quality Assurance Terms-Third Update (.pdf, 0.7 mb): PWL (also called percent conforming) is the percentage of the lot falling above the lower specification limit (LSL), beneath the upper specification limit (USL), or between the USL and LSL.
This quality measure uses the sample mean and the sample standard deviation to estimate the percentage of the population (lot) that is within the specification limits. This is called the PWL method, and is similar in concept to determining the area under the normal curve.
In theory, the use of the PWL method assumes that the population being sampled is normally distributed. In practice, it has been found that statistical estimates of quality are reasonably accurate provided the sampled population is at least approximately normal, i.e., reasonably bell shaped and not bimodal or highly skewed. Normality tests can be applied for verification.
Payment for pavement materials is tied to a more statistically accurate measure of quality that is based on controlling both the center and spread of the test results to determine how much of the material represented by those tests is within the specification limits. Many State highway agencies use methods to identify quality that only control either the mean or the variability of the test results.
After obtaining multiple random samples, PWL is computed starting with the mean and standard deviation of the samples tested. The mean and standard deviation are then used to compute the quality index and finally the quality index is converted to an "estimated" PWL using tables and computer software. PWL essentially estimates the total percentage of the material that meets the specification limits.
Once PWL is determined, the contractor is paid based on meeting the requirements. Properly established PWL will motivate the contractor to improve quality and innovate. When specification limits are set by good performance and achievable production, then PWL is a positive approach for achieving these goals.
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