The standards for the admission of expert testimony established in
Daubert v. Merrell Dow Pharmaceuticals,
Inc.,
I
As an employee in the shipping department at Tandy Rank Video (Tandy), Bal-kar Dhillon operated a stand-up forklift truck. The truck had been designed, manufactured, and distributed by defendant Crown Controls Corp. (Crown); defendant Crown Credit Company leased it to Tandy. *868 The truck model Tandy had was designed to be operated from a side-stance, standup position, at a maximum speed of six miles per hour. The operator compartment is fully enclosed with the exception of a rear opening used to get on and off the truck. From the side-stance position, the operator is able to observe both the forward and the reverse direction of travel; the forks are to the operator’s right and the opening in 'the compartment is to the operator’s left. The truck’s braking method operates in a way opposite of the traditional vehicle break — when the brake is depressed, the forklift will move; when the brake pedal is up, the brake is activated and the truck will not move.
On February 24, 1990, Dhillon was operating the forklift truck in reverse at about two miles per hour, heading for a telephone affixed to an I-beam so that he could respond to a page. For some reason, Dhillon allowed the forklift to get too close to the beam; fearing a collision, he attempted to shift the truck into the forward gear rather than apply the brake. When he shifted gears, the truck jerked, causing his left leg to slip out of the operator’s compartment and become pinned between the beam and the truck. Dhillon suffered severe and permanent injury to his leg and sued Crown in strict liability and negligence.
In his suit, Dhillon did not argue that there were any defects particular to the forklift he had been using; after the accident, the truck was used and inspected and found to be in proper working order. Instead, he claimed that the design of the truck was defective because the truck did not have a rear door (rather than just the opening) on the operator’s compartment that would have prevented his leg from falling off the truck. Additionally, Dhillon complained about the truck’s braking systems, the operator’s controls, and the inadequacy of the instructions and warnings.
The district court had removal jurisdiction based on diversity of citizenship between Dhillon (an Illinois citizen) and the defendants, which were all incorporated and with their principal places of business in Ohio; the amount of alleged lost wages alone exceeded $100,000. Prior to trial, the plaintiff sought the admission of evidence to be provided by two expert witnesses, John B. Sevart, a mechanical engineer, and Dr. Gerald Harris, a biom-echanical engineer. Both would have testified about an alternative design — an operator’s compartment with a rear door— and would have opined that Crown’s failure to equip the truck with a rear door was the proximate cause of Dhillon’s injuries and that adding a rear door would not have increased any risks to the operator. Sevart also would have stated that Crown was negligent in failing to provide appropriate warnings and instructions as to the availability of a rear door. Harris was prepared to add that a rear door would have prevented Dhillon’s injury. Dhillon had no expert who would have addressed the design of the braking system or the operator’s controls. The court, after applying Daubert, concluded that neither Sevart’s nor Harris’s testimony was entitled to pass through the evidentiary “gateway” that case establishes. Finding that without this expert testimony Dhillon could not possibly prevail, the court granted summary judgment for the defendants on March 13,. 2000, and dismissed the suit.
II
We review
de novo
whether the district court’s grant of summary judgment was proper. That is the case only when there is no genuine issue of material fact (or, as we sometimes put it, the record
*869
reveals that no reasonable jury could find for the moving party), and the moving party is entitled to judgment as a matter of law. See
Karazanos v. Navistar Int’l Transp. Corp.,
Under Federal Rule of Evidence 702 and the principles of
Daubert,
a district court judge is to act as a “gatekeeper” for expert testimony, only admitting such testimony after receiving satisfactory evidence of its reliability.
With regard to the first tier of analysis,
Daubert
offers a non-exclusive list of factors to aid judges in determining whether particular expert opinion is grounded in reliable scientific methodology. Among the factors articulated are: (1) whether the proffered theory can be and has been tested; (2) whether the theory has been subjected to peer review; (3) whether the theory has been evaluated in light of potential rates of error; and (4) whether the theory has been accepted in the relevant scientific community.
Daubert,
We could identify a number of problems with the testimony these witnesses were prepared to offer, but the most glaring among them is the lack of testing, or more generally the failure to take any steps that would show professional rigor in the assessment of the alternative designs (or, as the amended rule puts it, that the testimony is “the product of reliable principles and methods”). Although both experts wanted to assert that the truck design was defective because it
*870
did not include a rear door, neither expert has actually designed a model of a forklift truck with a rear door. Nor has either performed any tests of such a model to see if it is both economically feasible and just as safe or safer than the model without the door. In alternative design cases, we have consistently recognized the importance of testing the alternative design. See
Bourelle v. Crown Equip. Corp.,
It turns out that Sevart (at least) did conduct tests in 1991 and 1997 of forklift trucks with and without rear doors, but that does not improve Dhillon’s case. These tests were not put before the court, they were not otherwise made part of the record, they were performed after Sevart had already formed his opinion, and they were performed on a differently designed truck with a different operator’s position. Furthermore, Sevart did not explain how these past tests had led to his conclusions about the need for a rear door. Without a more detailed explanation of these tests and Sevart’s methodology, a court cannot possibly assess the tests’ reliability, and the defendant cannot attempt to duplicate the results. Of course, hands-on testing is not an absolute prerequisite to the admission of expert testimony, but the theory here easily lends itself to testing and substantiation by this method, such that conclusions based only on personal opinion and experience do not suffice. See
Cum-mins,
With regard to the inadequate warning/instructions claim, the same requirements apply. Sevart has not designed or suggested to the court an alternative warning that would have been appropriate or tested its effectiveness. See
Bourelle,
Of course,
Daubert
is a flexible test and no single factor, even testing, is dispositive. See
Kumho Tire,
For the second tier of the analysis, the Supreme Court has directed the courts to consider whether the testimony assists the trier of fact in understanding the evidence or determining a fact in issue.
Daubert,
This is not the first time that we have encountered a fork-lift accident in which the principal theory focused on the design of the lift. In
Bourelle,
Notes
. The amended rule allows the district court, after assessing the expert's qualifications, to admit expert testimony if "(1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.” Fed.R.Evid. 702.
