Can an Expert Give a “Scientific Guess”?

On appeal from the grant of summary judgment to a fleet company in a negligence action arising from a school bus fire, the plaintiff argued that the trial court erred when it excluded her expert’s opinion testimony and when it concluded that no genuine issue of fact remained as to the fleet company’s responsibility for the fire.


On the morning of January 19, 2016, the plaintiff noticed a burning smell and smoke coming from the 1993 Ford bus she was driving for Gwinnett County Schools. The heater was on at the time she noticed a burning smell and smoke. She took the bus to the county maintenance center where it was inspected by a county mechanic, who said the bus was low on antifreeze but found nothing else that concerned him. The mechanic added antifreeze, told the plaintiff that the bus had been low on antifreeze, which had caused the smoking, and said the bus was “ready to go.” Although the plaintiff testified that the mechanic spent less than five minutes conducting this inspection, the mechanic testified that it took between 10 and 30 minutes.

That night, a fleet company worker arrived to inspect buses (as the company was required to do every 20 working days) and found nothing wrong with the bus. The owner of the fleet company later told the County that the company had performed only a preliminary or “walk-around” inspection because they didn’t have the keys to the bus. The worker testified, however, and a printed report also showed, that he’d performed a more extensive inspection and had found a power steering fluid leak and an oil leak, resulting in a “yellow tag” on the bus. Had the fleet company found something more serious, the bus would have been “red-tagged” and taken out of service.

The following morning, January 20, 2016, was a cold one, and the plaintiff checked the antifreeze level and saw that it was full. She then started the bus, turned on the heater, and let it idle for 25 minutes or so, 15 of which were spent in her car while the bus warmed up. As she sat in the driver’s seat on returning from her car, she smelled the same odor as the day before and saw fire coming out from under the hood. She ran to the back of the bus, opened the emergency exit, and jumped out, hurting her knee and her back as she landed on the ground. The bus was destroyed in the fire.

Lawsuit Brought by the Bus Driver

In August 2017, the plaintiff brought this action against the fleet services company, alleging that it had failed to adequately inspect the bus as it was obligated to do under its contract with the County. The County, which wasn’t a party to this suit, hired a fire investigator to determine the cause of the fire.

The fire investigator was qualified as an expert and had extensive experience specifically with determining the cause and origin of bus fires. He reviewed maintenance records, interviewed the plaintiff and others, inspected the remains of the bus, and examined similar bus models. The expert found no evidence that the fire was caused by a fuel, electrical, or mechanical problem and therefore ruled those possibilities out as potential causes of the fire. Finding evidence that the cause of the fire was a coolant leak that had ignited on the turbocharger assembly and concluding that this was the only possible cause he “couldn’t eliminate,” he concluded that a coolant leak was the “most probable” or “most likely” cause of the fire. His report stated that “based on the driver smelling an odor” when the heating unit was on, a coolant valve “was likely leaking and spraying coolant in the engine compartment,” resulting in the fire. The fire investigator also noted damage to a valve through which coolant traveled and stated that “there’s two different places [on the valve that the coolant] could leak from.”

Based on his examination and analysis, the expert categorized a coolant leak ignition on the turbocharger as the “most probable” or “most likely” cause of the fire. In his report, he conceded that the “exact cause” of the fire couldn’t be determined “with absolute scientific certainty,” although he clearly stated that the coolant leak was the most probable cause. Under cross-examination at his deposition, the expert stated that since he could not state his opinion regarding the cause of the fire “with absolute scientific certainty,” that opinion was “basically a scientific guess on this because it is not absolute.” During that same deposition, he explained that of the possible causes, the coolant leak theory was the only one he couldn’t eliminate and that it was the most probable or most likely explanation. He stood by his report and confirmed that his opinion was based on the evidence that he reviewed, including the lack of evidence supporting any other plausible theory.

Seizing on the expert’s use of the words “scientific guess” and his inability to state his opinion to a reasonable degree of “scientific certainty,” the fleet company moved to exclude his causation opinion as speculative. It also moved for summary judgment on the ground that without the expert’s opinion, there was no other evidence creating a genuine issue of material fact regarding the cause of the fire. The trial court granted both motions, specifically holding that the expert’s opinion was inadmissible since he was unable to determine the cause of the fire with “absolute scientific certainty”, and concluding that “no other admissible evidence in the record establishes causation.” The plaintiff appealed.

The Court of Appeals Decision

Judge Clay D. Land of the Georgia Court of Appeals wrote that the first question before the trial court was whether the expert’s conclusion that a coolant leak was “most likely” the source of the fire had a reasonable scientific basis. OCGA § 24-7-702 (b) states that an expert witness “may testify in the form of an opinion” if:

  • The expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;
  • The testimony is based upon sufficient facts or data;
  • The testimony is the product of reliable principles and methods; and
  • The expert has reliably applied the principles and methods to the facts of the case.

By excluding the expert’s causation opinion due to his inability to state that opinion to a reasonable degree of “scientific certainty,” the trial court applied an incorrect standard that has previously been rejected by the Georgia Supreme Court. That court has made clear that an expert is not required to prove within a reasonable degree of scientific certainty his opinion of how an accident occurred. Instead, the appropriate standard for assessing the admissibility of an expert’s opinion “is not whether it is speculative or conjectural to some degree, but whether it is wholly so.”

Judge Land explained that as the trial court held, this is not a case where the expert failed to utilize an appropriate scientific methodology in the formation of his opinion. He did in fact utilize such a methodology. This is a case where the causation expert provided a plausible, scientifically supportable explanation for the most probable or most likely cause of the fire while recognizing and acknowledging that he could not state that opinion with absolute certainty. Because the expert wasn’t required to state that opinion with absolute certainty, the trial court’s exclusion of it on this basis was erroneous.

“Although an expert [may lack] direct evidence of the cause of [a] fire, the expert may rely on circumstantial evidence to support his theory,” Judge Land wrote, quoting a Court of Appeals decision from 2017. That’s what the expert did here, the judge said. Even if his opinion was based on inadequate knowledge, it didn’t mandate the exclusion of his opinion but, rather, presented a jury question as to the weight that should be assigned to his opinion.

Judge Land said that when read in context, the expert’s statement that his opinion was his best “scientific guess,” it was simply his recognition of the fact that the “exact cause” of the fire couldn’t be determined with absolute certainty. That’s not the standard for the admissibility of expert testimony. The expert’s opinion was based on the following:

  • His examination of the remains of the bus;
  • His observation of the flame patterns and damage to the bus engine;
  • Evidence that the bus had been leaking coolant;
  • Evidence that the plaintiff smelled a burning odor on both occasions while the bus’s heater was running; and
  • Evidence that coolant valves on two similar buses showed similar signs of leakage.

Based on all of this evidence, the expert’s opinion that a coolant leak was the “most probable” or “most likely” cause of the fire was admissible. The trial court’s exclusion of this opinion testimony was based on an erroneous legal standard, and the Court of Appeals reversed on this ground. Early v. Morgan Fleet Services, Inc., 2023 Ga. App. LEXIS 203 (Ga. App. May 16, 2023).

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