Understanding Slip Resistance Testing Standards

The first post of Expert-aganza ’16 discussed trip and falls, this post discusses trip and fall’s fraternal twin, slip and fall (or perhaps brother by a different mother.  Either way, they are closely related).



Liability is often difficult to prove for slip and fall, personal injury plaintiffs.  Some attempt to address that problem by hiring someone to use a tribometer to measure the co-efficient of friction in the area of the fall.[1]  The theory is that, assuming the source of the hazard that caused the fall cannot be identified to help prove liability, the co-efficient of friction on the floor may have been lower than it should have been.  This means the floor was slipperier than it should have been or has a low “slip resistance.”  ASTM F1646 defines “slip resistance.”  It is the “relative force that resists the tendency of the shoe or foot to slide along the walkway surface. Slip resistance is related to a combination of factors including the walkway surface, the footwear bottom, and the presence of foreign materials between them.”

The co-efficient of friction is most commonly “measured” by using a tribometer.  There various tribometers in use, such as the English XL Variable Incidence Tribometer (VIT) or a Brungraber Mark III.[2]  When evaluating an expert report that concerns the co-efficient of friction and relies upon these there are multiple topics to address.  This post addresses some of them.

  • Is 0.5 really a valid standard?

The plaintiff’s expert is likely to describe how the co-efficient of friction is measured on a scale from 0.0 to 1.0 and anything below 0.5 is a failure.  The problem is there is no standard setting 0.5 as the universally accepted standard for all flooring types, under all conditions.  For a very detailed explanation of why, I recommend reading The Changing World of Slip-and-Fall Defense from the February, 2013 edition of DRI’s For the Defense.

  • Is the expert qualified to use the device?

This question is usually easily answered, especially because the English XL manufacturer holds regular classes to those who want the certification.  The next question becomes whether the expert followed the manufacturer’s testing guidelines.

However, occasionally someone is not certified.  This issue arose in Page v. Supervalu, Inc. where the expert was not certified to use an English XL.[3]  Given the lack of a certification, the expert used an alternative device, the Brungraber Mark IIIB.  However, the expert’s training on this device was also lacking.  The expert had not attended any classes about how to use the device and had no other qualifications to measure slip resistance.  “Considering Mr. Parsons’ lack of specific education and the lack of experience using the Brungraber Mark IIIB (which he used for the first time, outside the laboratory, during his on-site inspection in this case),” he was excluded.

The expert was also excluded in another area.  “Although Mr. Parsons has extensive experience in retail operations safety and is apparently conversant with industry standards regarding the safety of patrons and accident prevention, the court nevertheless finds the ‘area of grocery store maintenance and safety’ does not qualify as nor constitutes scientific, technical or specialized knowledge.”

  • Did the expert calibrate the device?

All measurements are only as good as the calibration on the instrument used to measure.  This applies to slip resistance testing too.  Calibration was the primary reason the expert was excluded in Kill v. City of Seattle.[4]

Kill slipped and fell on a small utility cover.  She hired an expert to use the English XL to measure the co-efficient of friction.  The expert measured it twice: February, 2011 and June, 2013.  The court determined the standard that governed the device was ASTM F2508.  It then began cataloging the problems with the expert’s data.

ASTM F2508 required a two stage calibration process.  First, the device’s calibration must be provided by a third party.  This means the expert cannot be the only one to attest to the device’s calibration.  To be certified as calibrated, the tribometer (1) “must rank the coefficient of friction for each of four reference surface tiles in the correct order; and (2) it must produce statistically significant results, using the mean and standard deviation, for all adjacently ranked surface tiles.”  If the tribometer passes, the manufacturer then creates “a validation report specifying the 95th percentile confidence interval for each reference surface tile.”

This validation report is important.  Before the end user uses the tribometer for testing a surface, the user is required “to perform calibration of their tribometer to ensure valid test results.  The tribometrist must measure each of the four reference surface tiles and compare the results to the 95th percentile confidence interval specified in the manufacturer’s validation report.”  If the results are not within the accepted range, the device is out of calibration and unreliable.

All of this was relevant in Kill because the February, 2011 test produced a 0.32 coefficient of friction, but the June, 2013 test produced a 0.21 coefficient.  The day before the June, 2013 test, the expert had attempted to verify the tribometer’s calibration but none of the verification tests were within the 95th percentile, meaning the tribometer failed.   The trial court excluded the expert because there was no sufficient explanation as to how the testing results could be so different.  The expert then sent her tribometer to the manufacturer, who also found the device was not within the 95th percentile.

The appellate court affirmed exclusion.  The difference between the two test was “0.14, almost 15 percent of the entire coefficient of friction scale, which ranges from 0.0 to 1.0. This is not a minor variance.”  Second, the expert had not explained how what she measured was substantially similar to what the plaintiff encountered.

[T]here is no documentation in the record of the rim’s condition on the day Kill slipped, other than it was wet.   Gill poured water on the rim for her second test, but was able only to speculate that some type of surface contaminant must have caused the variation in test results. Without more evidence, Gill could not establish substantial similarity between her two tests, nor between the test conditions and the condition of the rim when Kill fell. As Gill’s test results and explanation demonstrate, surface contamination can have a dramatic impact on the slipperiness of the metal rim.

Finally, the court rejected the idea that a 0.5 coefficient was an “absolute threshold for safety.”

The City provided copious evidence that tribometers are effective to measure only relative slipperiness, not absolute slipperiness. Different tribometers, both across models and within models, can give significantly different readings for the same surface. Thus, a tribometer can accurately determine whether one surface is more or less slippery than other surfaces measured by that same tribometer, but it cannot give an objective measurement of a surface.



  • Is the expert applying the correct standard?

Kill relied upon F2508 and I suspect most courts currently would.  However, some experts may still use its predecessor, F1679.  Michaels v. Taco Bell Corp.[5] addressed which standard applied during the period of time between F1679 and F2508.

The plaintiff’s expert asserted he used ASTM F1679 to calibrate his VIT for his testing on May 20, 2011.  As Kill noted though, F1679 was withdrawn in September, 2006 and was not replaced until F2508 was adopted in March, 2011.  Taco Bell argued the expert should have used F2508, not F1679.  Unlike Kill, the expert argued F1679 had been appropriate at the time the testing occurred.  “Whatever its shortcomings, however, Karlin’s testimony establishes that at the time of his testing, F1679 was in general use to calibrate tribometers.”  The court also concluded F2508 had not yet gained general acceptance only two months after it was promulgated.  The expert survived that challenge given the timing, but I suspect the result would be different if the same issue arose now.

Instead, the expert was excluded because his methods were unreliable.  He criticized the floor tiles themselves but failed to gather critical information.

He concludes that”[f]loor tiles with measurably better slip-resistance were available for this use,” but nowhere in his report does Karlin identify the manufacturer and model of floor tile actually used in the Taco Bell restaurant where Plaintiffs fall occurred. He provides no information about the manufacturer’s specifications regarding the tested tile’s slip resistance or whether the tested tile, as manufactured, met government or accepted industry standards for slip resistance. Also absent is any identification of the other floor tiles he claims were better and whether these tiles were available for use at the time the subject floor tiles were purchased and installed in the Taco Bell restaurant.

Equally critical on this point is Karlin’s deposition admission that he did not know even what kind of tile was present in the Taco Bell restaurant when he tested it. When asked at deposition, he replied, “it’s a light brown or mauve 8-by-8 tile” but did not know the kind of tile or whether it is a tile generally used in commercial facilities. Karlin failed to satisfactorily explain why, when he had adequate opportunity to acquire and incorporate this knowledge into his testing, he failed to do so. That the information was available to Karlin is undisputed. In its brief supporting its motion and at hearing on its motion, Defendant’s counsel represented, and Plaintiffs counsel did not refute, that in February 2011, Plaintiff deposed Michael Singhose, a Taco Bell architect, who identified the type of tile used and discussed the rationale for its use at the restaurant where Plaintiff fell.

Then his testing failed to reasonably replicate the conditions the plaintiff encountered.  “Karlin tested the tile floor using soap solution, but he testified in deposition that he did not know whether any soap solution was used to mop the floor on the day of Plaintiffs fall” and acknowledged an employee stated no soap was used that day.   “Karlin also tested the tiles after creating a small puddle of water on the floor, but neither the written record or the still photos taken from the surveillance CD depicting Plaintiffs fall show a puddle on the floor where Plaintiffs fall occurred.”  He also failed to consider the actual condition the plaintiff encountered.  “And, a soda spill had occurred and had been mopped up in the area of Plaintiffs fall just before it occurred, but none of Karlin’s testing included the use of soda, whether before or after it had been mopped up.”  The last error was that Taco Bell had described its “standard process for cleaning floors at the restaurant where Plaintiff fell and identified the floor cleaner solution used, yet Karlin’s report contains no mention of this information and no explanation why such information was not relevant….”

It got worse.  The expert’s test results indicated the dry floor failed testing, but the wet floor passed.  “Even to a lay person this comparative result is counterintuitive, yet when asked about the result during his deposition, Karlin simply replied, ‘I don’t know.’”  The expert was excluded.

The End

Remember, science continually evolves just like law and all of this information can change as new processes are designed and refined.  However, slip resistance opinions are not as simple as measuring to 0.5.


[1] The co-efficient of friction is a way of measuring how slippery a surface is.
[2] Others use the horizontal pull slipmeter, at their peril, as discussed in the November 23, 2015 post,
[3] No. WGC-14-1508, 2015 U.S. Dist. LEXIS 38198, 2015 WL 1439572 (D. Md. Mar. 26, 2015).
[4] No. 70767-1-I, 2014 Wash. App. LEXIS 2094 (Wash. App. Aug. 25, 2014).
[5] Civ. No. 10-1051, 2012 U.S. Dist. LEXIS 140283, 2012 WL 4507953 (D. Or. Sept. 27, 2012).