Expert Witness Testimony – Properly Stating an Expert Opinion

Expert Witness Testimony – Properly Stating an Expert Opinion
Post By Alex Babitsky – SEAK, Inc.

One of the primary responsibilities of an expert is to render an opinion that will assist the trier of fact. This opinion needs to be stated in a legally sufficient manner and must be based upon reliable facts, data, and methodology. Experts can expect to be closely questioned on their opinions, how they were formed, and the facts and data upon which they are based.

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Properly Stating an Expert Opinion

In most civil cases, the legal requirements for stating an expert opinion is related directly to the burden of proof that exists. That burden of proof is a “preponderance of the evidence,” “more likely than not,” or “more than 50% likely.” This is a much lesser burden of proof than the “beyond a reasonable doubt” standard with criminal cases.

The expert’s opinion must satisfy the “preponderance of the evidence” burden of proof. This means that the expert must opine that it is more probable than not (there is more than a 50% probability) that his opinion is correct. Thus, an expert witness may give testimony in terms of an opinion that something could, or would, produce a certain result. The theory for admitting opinion testimony of this nature into evidence is that an expert witness’s view regarding probabilities is often helpful in the determination of questions involving matters of science or technical or skilled knowledge.

The facts or scientific principles on which experts base their opinions must be sufficient to support reasonably accurate conclusions. Expert witnesses will not be barred from expressing opinions merely because they are not willing to state their conclusions with absolute certainty. However, expert opinions, if not stated in terms of the certain, must at least be stated in terms of the probable and not merely of the possible. The test of whether an expert witness testimony expresses a reasonable probability is not based upon the use of “magic words” but is determined by looking at the entire substance of the expert’s testimony.

Although no magic words are required, certain phrases are commonly used by expert witnesses to express the idea that their opinions are based on at least a 51% probability. These phrases include:

• “based on a reasonable degree of medical certainty,”

• “based on a reasonable degree of scientific probability,”

• “based on a reasonable degree of scientific certainty,”

• “based on a reasonable degree of medical probability,” and

• “more likely than not.”

When an expert does not express the concept that she is at least 51% sure of her opinion, the opinion might be excluded by the judge. Thus, it is important to state an opinion in a way that clearly communicates that it is based upon a reasonable degree of probability and not just a mere possibility or speculation.

 Conclusion

Experts need to be prepared to express and deliver their opinions in a legally sufficient way. Speculation and guessing are not permitted. Opinions need to be based upon reliable facts and methodology or they may be excluded from evidence. Cross-examination will challenge the expert’s opinions, methodology, and the facts and data upon which the opinion is based.  The expert should be prepared for such challenges.

One of the primary responsibilities of an expert is to render an opinion that will assist the trier of fact. This opinion needs to be stated in a legally sufficient manner and must be based upon reliable facts, data, and methodology. Experts can expect to be closely questioned on their opinions, how they were formed, and the facts and data upon which they are based.

Properly Stating an Expert Opinion

In most civil cases, the legal requirements for stating an expert opinion is related directly to the burden of proof that exists. That burden of proof is a “preponderance of the evidence,” “more likely than not,” or “more than 50% likely.” This is a much lesser burden of proof than the “beyond a reasonable doubt” standard with criminal cases.

The expert’s opinion must satisfy the “preponderance of the evidence” burden of proof. This means that the expert must opine that it is more probable than not (there is more than a 50% probability) that his opinion is correct. Thus, an expert witness may give testimony in terms of an opinion that something could, or would, produce a certain result. The theory for admitting opinion testimony of this nature into evidence is that an expert witness’s view regarding probabilities is often helpful in the determination of questions involving matters of science or technical or skilled knowledge.

The facts or scientific principles on which experts base their opinions must be sufficient to support reasonably accurate conclusions. Expert witnesses will not be barred from expressing opinions merely because they are not willing to state their conclusions with absolute certainty. However, expert opinions, if not stated in terms of the certain, must at least be stated in terms of the probable and not merely of the possible. The test of whether an expert witness testimony expresses a reasonable probability is not based upon the use of “magic words” but is determined by looking at the entire substance of the expert’s testimony.

Although no magic words are required, certain phrases are commonly used by expert witnesses to express the idea that their opinions are based on at least a 51% probability. These phrases include:

  • • “based on a reasonable degree of medical certainty,”
  • • “based on a reasonable degree of scientific probability,”
  • • “based on a reasonable degree of scientific certainty,”
  • • “based on a reasonable degree of medical probability,” and
  • • “more likely than not.”

When an expert does not express the concept that she is at least 51% sure of her opinion, the opinion might be excluded by the judge. Thus, it is important to state an opinion in a way that clearly communicates that it is based upon a reasonable degree of probability and not just a mere possibility or speculation.

 Conclusion

Experts need to be prepared to express and deliver their opinions in a legally sufficient way. Speculation and guessing are not permitted. Expert opinions need to be based upon reliable facts and methodology or they may be excluded from evidence. Cross-examination will challenge the expert’s opinions, methodology, and the facts and data upon which the opinion is based.  The expert should be prepared for such challenges.