From Rose v. Truck Centers, Inc., 2010 U.S. App. LEXIS 16396 (6th Cir. Aug. 6, 2010):

In order to qualify as an expert under Rule 702, a witness must establish his or her expertise by reference to "knowledge, skill, experience, training, or education." Fed. R. Evid. 702. Although this requirement is typically treated liberally, a witness is not an expert simply because he claims to be. *** "The issue with regard to expert testimony is not the qualifications of a witness in the abstract, but whether those qualifications provide a foundation for a witness to answer a specific question." Berry v. City of Detroit, 25 F.3d 1342, 1351 (6th Cir. 1994). In Berry, the Court delineated by analogy the kinds of testimony witnesses are qualified to give based upon their expertise — scientific or technical:

The distinction between scientific and non-scientific expert testimony is a critical one. By way of illustration, if one wanted to explain to a jury how a bumblebee is able to fly, an aeronautical engineer might be a helpful witness. Since flight principles have some universality, the expert could apply general principles to the case of the bumblebee. Conceivably, even if he had never seen a bumblebee, he still would be qualified to testify, as long as he was familiar with its component parts.

On the other hand, if one wanted to prove that bumblebees always take off into the wind, a beekeeper with no scientific training at all would be an acceptable expert witness if a proper foundation were laid for his conclusions. The foundation would not relate to his formal training, but to his firsthand observations. In other words, the beekeeper does not know any more about flight principles than the jurors, but he has seen a lot more bumblebees than they have.

Id. at 1349-50.

Smith testified that he was an expert in the areas of "[t]ruck mechanical issues, electronic control modules, driver issues, logs, hiring practices, [and] heavy hauling" and an expert in steering gears.*** Defendant argues that Smith's education and experience do not provide him with the requisite qualifications to offer opinion on product defect or causation. Plaintiffs counter that the absence of a proffered expert's qualifications "almost always bears on the weight that the jury should assign to the [expert's] testimony and not on the admissibility of the testimony itself."

The district court concluded that Smith's testimony exceeded the scope of his expertise. Specifically, the court pointed to Smith's opinion regarding the cause of the accident and Smith's conclusion that the steering gear was defective. The district court found that Smith lacked the formal education, training, and experience to offer an opinion regarding whether the steering gear was defective and concluded that Smith failed to demonstrate that he possessed any more knowledge about mechanical engineering principles than an average juror.

However, as this Court has stated, "Rule 702 should be broadly interpreted on the basis of whether the use of expert testimony will assist the trier of fact." *** The record in this case shows that Smith was qualified as an expert since his testimony would certainly assist the trier of fact in understanding the evidence. Smith's experiences as a mechanic give him specialized knowledge in the areas of truck mechanics and steering gears and, in his deposition testimony, enabled Smith to thoroughly discuss the process of torquing the steering gear bolts and how a misstep in that process could affect the integrity of the steering gear and possibly result in an automobile accident. Accordingly, we conclude that Smith was qualified as an expert, at least for some purposes, pursuant to Rule 702. ***

Despite Smith's qualifications, however, we conclude that the district court did not abuse its discretion in excluding Smith's opinion because, as explained below, Smith's opinion was unreliable. ***

In this case, Smith admitted that he engaged in no testing to replicate his theory of what occurred during the accident. He also could not identify any tests by another person demonstrating how the bolts would suddenly loosen in the manner he describes after thousands of miles of normal operation. Smith admitted that he knew of no other instance where the bolts on a steering gear came loose in the manner commensurate with his theory. Moreover, vibration is a critical component of Smith's theory, yet he conducted no testing to determine the vibration level of Plaintiffs' truck or similar trucks.

Smith first inspected the steering gear at Plaintiffs' home in November 2006, six months after the accident. *** Smith's conclusions in his report are based on the assumption that on the day he inspected the steering gear, the bolts were at the precise degree of looseness that they were at the time of the accident. However, as the district court correctly noted, a photograph of the steering gear taken in July 2006 indicates that the position of the bolts was altered between the time of the accident and Smith's examination. ***Because the evidence underlying Smith's conclusion regarding defect and causation had been altered prior to his inspection and evaluation, the district court did not abuse its discretion in excluding Plaintiffs' expert's testimony.