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Highway Quality Compendium

Percent Within Limits: The Quality Measure of Choice

Meet the new quality measure of choice. Now available from the Federal Highway Administration (FHWA) is a 1-day introductory workshop on Percent Within Limits (PWL). As the workshop demonstrates, PWL is not just business as usual. "Generally the highway industry does not use an accept/reject model for evaluating contractors' work," notes Jim Walls of FHWA. "Rather highway agencies accept what is produced and pay accordingly, using payment systems that have incentives and disincentives." In contrast, the PWL model encourages highway contractors to produce consistent quality work and then rewards that work by tying payment to a statistically valid measure of quality.

The PWL workshop debuted February 8, 2006, in Raleigh, North Carolina. The workshop provides an overview of quality measures in general, details on how PWL works, and specifics on computing PWL. Also covered are payment issues and implementation steps and resources. Hands-on exercises give participants the opportunity to compare and contrast quality measures and compute PWL for sample materials data. A refresher module on basic statistical concepts is also included, covering such topics as probability, standard deviation, and sample size. The workshop is designed for State highway agency and FHWA division personnel responsible for developing and overseeing quality assurance specifications, as well as pavements and materials engineers.

More than 20 staff from the North Carolina Department of Transportation (NCDOT) attended the workshop. "There is a lot of interest in learning more about PWL, but also a lot of questions," notes Cecil Jones, State Materials Engineer for NCDOT. "PWL gives you a mathematical way to evaluate quality, but we have to make sure it's implemented in a way that's user friendly," says Shannon Sweitzer of NCDOT.

Quality measures for roadway projects are designed to obtain a more uniform product, increase service life, reward contractors for excellent work, and ensure that payment is appropriate to the product received. "With PWL, we're focusing on the degree to which a product or service conforms with a given requirement," says Ewa Flom of FHWA. Using PWL, a State can set specification limits and then determine the acceptable quality level for a job, which is the PWL value at which the contractor should receive 100 percent payment, and the rejectable quality level. The rejectable quality level is the PWL value at which the material or construction is unacceptable. Specification limits should be based on expected performance and contractor capabilities, and linked to life-cycle costs.

"PWL is a more efficient quality measure, allowing quality to be assessed with the lowest number of tests," says Walls. And unlike such quality measures as computing an average from material samples, PWL captures both the mean and standard deviation in one measure. This encourages contractors to produce a more uniform product.

To use PWL, material is sampled and tested and then analyzed statistically to determine the total estimated percentage of the lot that meets the specification. This is the PWL estimate. A PWL of 98.3, for example, means that 98.3 percent of the material meets the project specification. The workshop covers possible pay plans that can then be used, including stepped, continuous, and multilinear plans.

The workshop concludes by looking at challenges that have to be addressed when implementing PWL, including the need to define goals and expectations, understand best practices, reach agency consensus, and get top management and industry support. Resources available to assist with implementation include such FHWA publications as Optimal Procedures for Quality Assurance Specifications (Publication No. FHWA-RD-02-095) and Evaluation of Procedures for Quality Assurance Specifications (Publication No. FHWA-HRT-04-046). These publications are available online at www.fhwa.dot.gov/pavement/pub_listing.cfm.

"The workshop was a good introduction to PWL. It enabled us to learn more about PWL and what it is all about," says Wiley Jones of NCDOT. On the day following the workshop, some NCDOT staff met with Dennis Dvorak of FHWA's PWL team to more specifically discuss applying PWL in North Carolina. "We went over practical guidelines for using PWL, such as how to set up limits and what to look for," says Randy Pace of NCDOT. "It gave us an opportunity to ask questions and get more specific examples. There is a lot of interest in learning more about PWL." North Carolina is now looking at scheduling another PWL workshop.

For more information on PWL or scheduling the workshop in your State, contact one of the FHWA PWL team members listed below. The PWL team members are also available to provide technical assistance in conjunction with the workshop.


Dennis Dvorak, Resource Center
708-283-3542 (email: dennis.dvorak@fhwa.dot.gov)

Ewa Flom, Office of Pavement Technology
202-366-2169 (email: ewa.flom@fhwa.dot.gov)

Lee Gallivan, Office of Pavement Technology
317-226-7493 (email: victor.gallivan@fhwa.dot.gov)

Jim Walls, Resource Center
410-962-4796 (email: jim.walls@fhwa.dot.gov)

Reprinted from Focus, March 2006.

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Updated: 06/27/2017
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