I just had a case on Friday where the Commonwealth brought an expert medical doctor. To jump to the end, my client was found not guilty. Unlike most of my cases as a Bucks County criminal lawyer, this case was in Philadelphia.
The expert was smart and intelligent. She testified credibly and honestly. She stuck to what they call "a reasonable degree of professional certainty." What does this mean? It revolves around Pennsylvania Rules of Evidence 702 and McMahon v. Young, 442 Pa. 484, 276 A.2d 534 (1971), which created "the requirement that an expert’s opinion must be expressed with reasonable[professional] certainty."
The Pennsylvania Supreme Court, in Collins v. Hand, ruled that "[n]o matter how skilled or experienced the witness may be, he will not be permitted to guess or to state a judgment based on mere conjecture."
So, essentially, the expert must testify that in their professional opinion the result in question came from the cause alleged. But, some language can render the opinion with less than the requisite certainty. In Corrado v. Thomas Jefferson University Hospital in 2001 that an opinion that is "more likely than not" is insufficient. Other cases have held that "in all likelihood", "could have", "possible", or "could very properly account for" is short of the requisite certainty.