Question: How certain does an expert need to be to give an opinion at trial?
Answer: It depends. See below. The standard for admissibility of expert witness evidence and the degree of certainty with which that evidence must be offered by the expert in order to support a verdict. There is no threshold level of certainty or conclusiveness is required in an expert’s opinion as a prerequisite to its admissibility. Assuming the subject matter is one which is appropriate for expert testimony and that a proper foundation has been laid, the expert’s opinion or conclusion that, in the context of the facts before the witness, a particular proposition is “possible,” “could have been,” “probable,” or “reasonably certain” all serve to assist the finder of fact in intelligently resolving the material factual questions. The degree of certainty in which an opinion or conclusion is expressed concerns the weight to be accorded the testimony, which is a matter for the jury to resolve. Notwithstanding probative value and admissibility of an expert’s opinion which falls short of “reasonable scientific or medical certainty,” an opinion which lacks reasonable certainty or probability is not sufficient evidence by itself to support a verdict. Of course, an expert’s opinion that something is “possible” or “could have been” may be sufficient to sustain a verdict or award when it has been rendered in conjunction with other evidence concerning the material factual question to be proved.