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23 300 250 Coefficient of Variation, % 200 150 B/L W/L 100 50 0 0 2 4 6 8 10 12 14 Average Percent F&E, % Figure 3. Pooled coefficient of variation for F&E tests versus average for 51 ratio (41). conducted tests by the same operator on the same sample The AASHTO Materials Reference Laboratory has also using the same proportional caliper should not differ by more compiled statistics for the precision of flat and elongated than 73.9% of their average. The multilaboratory coefficient aggregate tests performed on proficiency samples No. 117 of variation has been found to be 35.3%. Therefore, results and 118 (43). The results are shown in Table 6. The results of two laboratories on identical samples of an aggregate are based on testing at the 51 ratio. Similar to the research should not differ by more than 99.9% of their average. For by Prowell and Weingart (41), the results indicate the comparison, the single operator and multilaboratory coeffi- tremendous variability of the test method. cients of variation for the 21 ratio are 9.1% and 15.0%, respectively. A precision statement was not prepared for the 51 testing 2.3.4 Summary of Research Related to F&E ratio because of the variability in the standard deviation and the coefficient of variation as a function of the mean. The high A limited number of studies have been conducted to relate within- and between-laboratory standard deviations and coef- the percentage of F&E to performance since the implemen- ficients of variation call to question the value of using ASTM tation of the Superpave method. None of the studies have D4791 for a specification limit unless the 21 ratio is used. addressed the relationship between F&E and performance 7 6 Standard Deviation, % 5 4 B/L 3 W/L 2 1 0 0 2 4 6 8 10 12 14 Average Percent F&E, %