The Proficiency Sample Program allows participants to evaluate the reliability of their testing by comparing their test results to a population of test data generated by all participants. Specified routine tests are performed in accordance with standard Arizona and AASHTO test methods by each participating laboratory on carefully prepared samples of highway construction materials and the test results reported to ADOT for review and analysis.
Regular participation in the program is required for all laboratories performing any testing activities for the department, as specified in the System for the Evaluation of Testing Laboratories.
Proficiency samples are carefully prepared to be as homogenous as possible to minimize the effect of material variability in evaluating the results. Each sample is sequentially numbered, and using random numbers, a set of samples is allocated to each participant. To permit an estimate of single-operator precision, instructions are given for a single test operator to conduct all repetitions of an individual test method; however, it is not required that the same person conduct all test methods prescribed for a set of proficiency samples. Typically, the material types and routine tests performed are as follows:
- Soil: gradation, Atterberg limits (PI), pH, resistivity, soluble salts, moisture-density relations.
- Fine aggregate: gradation, sand equivalent, fine-specific gravity, absorption.
- Coarse aggregate: gradation, specific gravity, absorption, L.A. Abrasion, unit weight, fractured faces, flakiness index.
- Asphaltic concrete: asphalt content, maximum theoretical specific gravity/density (Rice), Marshall stability/flow, Marshall compaction/density, gyratory compaction/density, moisture content, gradation of mineral aggregate.
- Portland cement concrete: 7 and 28 day compressive strengths of prepared cylinders.
- Asphaltic concrete mix design: immersion compression.
- Asphalt cement/binder: rotational viscosity, pressurized aging, flash point, bending beam rheometer, direct tension, dynamic sheer rheometer.
- Emulsified asphalt: Saybolt-Furol viscosity, percent (%) residue, percent (%) uncoated particles, absolute viscosity, sieve test.
A final report summarizing the results of the analysis is issued for each proficiency sample. The final report presents a statistical summary of results for the population of test data and a tabulation of each laboratory's individual data. Statistical characteristics presented are averages, standard deviations, coefficients of variation, Z-scores and performance ratings. The Z-score is equal to the number of standard deviations the data departs from the population mean. All data submitted is initially reviewed and analyzed. Invalid data is eliminated, and the remaining data is reanalyzed and presented in the proficiency sample final report. A single low rating, or a pair of low ratings, is not considered significant. A continuing trend of low ratings for a test characteristic should cause a laboratory to investigate its equipment and test methodology.
The "History of Z-Scores" charts accompany each proficiency sample final report. These charts show a laboratory's performance trend for each test characteristic. In addition, scatter diagrams are included in each proficiency sample report for each test characteristic. A scatter diagram shows each laboratory's reported results as a point on the graph, relative to the population averages for that test.