The XL Achieves Precision and Bias
A Report of Round Robins and Workshops Conducted with the XL Slipmeter
Last update by William English July 26, 2003


A series of round robins have been conducted with the English XL slipmeter (which is also known in ASTM circles as the VIT or Variable Incidence Tribometer), beginning with the F13 slipmeter workshop conducted at ASTM Headquarters at the June 1997 meeting of ASTM F13 on Footwear Safety and Traction. This first run at precision and bias was an attempt to work out operating procedures and statistical protocol in compliance with ASTM E691 on precision and bias (guidelines for statistical validation of ASTM test procedures.) In this initial all-day testing workshop both the Brungraber, Mk II (PIAST) and the English XL (VIT) were operated by a number of volunteer testers. There were three kinds of test surfaces, and for the VIT we tested using the traditional 400 grit sandpaper for conditioning of the test feet and again using 180 grit, which appeared to give us a slider pad surface texture that was more in line with actual heel wear. As far as the XL slipmeter is concerned, the conclusions of this experiment were that (1) the 180 grit paper would become standard for XL test foot preparation, and (2) that we needed more uniform test surfaces. There was wide variability on the textured vinyl surface, and there were two significant outliers on all three kinds of surfaces.

There was also a limited workshop for interested XL slipmeter users at the January 1998 F13 meeting in San Diego where they got together informally on Sunday evening to test the same surfaces with every participant observing and critiquing everybody else’s operating technique. I was not able to attend that meeting, but reportedly progress was made and the outliers were brought into the fold as far as test results are concerned. Jim Flynn of J2 Engineering in Fresno, CA was the denmother of all of these experiments.

Following that first workshop, a couple of "mail-order" round robins were conducted in which the same three test surfaces were circulated to seven different labs and everybody used the same test foot. The purpose was to establish that the meters produced repeatable results in different labs.

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In the third "mail-order" round satisfactory precision and bias were achieved, and David Underwood, PhD, produced a statistical evaluation using ASTM E691 software, including the following summary, which compares the XL’s performance with the other slipmeters for which precision and bias figures are published in their respective ASTM standards.

For the VIT
Dry reproducibility: std. deviation 0.02, wet reproducibility: std. deviation 0.03 (testing performed on 3 surfaces using 6 testers) This fine precision compares favorably with traditional slipmeters: