Establishing Credibility: Words and Phrases to Avoid as an Expert Witness

As every attorney, judge, and—most importantly—jury knows, the most critical characteristic of an expert witness offering testimony is their credibility. An expert could be the most qualified in their field, but it means nothing if the jury does not believe them. Especially in cases where opposing expert testimonies are facing off, the jury’s interpretation can become a credibility challenge.

Anjelica Cappellino, J.D.

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— Updated on August 18, 2021

Establishing Credibility: Words and Phrases to Avoid as an Expert Witness

The Basis of Credibility

The importance of credibility is so fundamental in the litigation process that researchers have attempted to quantify its elements in a near scientific manner. Psychological researchers have empirically identified and broken down four components of courtroom credibility—trustworthiness, knowledge, confidence, and likeability. Notably, an expert’s knowledge is just one of several contributing factors to their overall credibility. Further, because a jury is not equipped to opine on the underlying scientific merits of an issue alone (why an expert in the field is necessary in the first place), an expert’s ability to sway a jury with a scientific fact is more likely determined by their ability to teach and persuade. Proving one’s credibility as an expert is a multi-faceted, complex undertaking that, try as one might, is less of a science but more of an art. But experts can evade pitfalls when giving testimony (or providing written reports of their opinions), just by avoiding certain words or phrases that tend to hamper credibility. Below are just a few examples of those to avoid.

Absolute Statements

An expert witness should be confident in their testimony, but not infallible. As part of the Daubert factors for admitting expert testimony (which are also codified in Rule 702 of the Federal Rules of Evidence), the witness’s opinion should be based on sufficient fact or data. Testimony should also be the product of reliable principles and methods, which were reliably applied to the facts of the case. The advantage of such a rule, and the scientific method in general, is that an expert’s opinion is based on tried and true methodologies and testing. That does not mean, however, that scientific theories or opinions are not subject to change. If such were the case, scientific research would stagnate and remain unadvanced. Therefore, an expert witness should avoid words or phrases that suggest their opinion is beyond reproach. Words such as “always,” “never,” or “110% percent,” can quickly be used against an expert during cross examination if opposing counsel challenges their figurative usages of such words as literal exaggerations.

For example, if there is an insignificant margin of error in an expert’s testing, it might be tempting to “round up” and state a 100% accurate testing rate exists. But since such a statement could be disproven on cross examination, the minor lie is not worth the significant damage to credibility it may cause.

Hedging Language

At the same time, an expert should be wary to use any language that suggests they are proceeding cautiously or evasively in stating their opinions. Hedging language may sound like it is solely based on emotion or point of view, i.e., “I believe,” “I feel,” “I suppose,” “I think,” etc. Alternately, it may come up as the overuse of adverbs that tend to express hesitation such as “probably,” “possibly,” or “conceivably.”

It may seem like there is no middle ground between hedging language and absolute statements. But the most important way to avoid either type of phrasings is by couching every opinion on the facts and evidence at hand. For example, if an expert’s study concluded their opinion is correct 90% of the time, it is safer to begin the statement with  “Because such results were indicated 90% of the time…” instead of the wavering “I believe…” which also avoids language that suggests 100% accuracy.


The rule for assumptions is simple—don’t make them. By definition, an expert witness is needed when the jury could use help understanding the facts at issue. An expert should be intimately familiar with the facts of the case and by the time they are on direct examination, they should be adequately prepared to express their opinions on the evidence at hand. All statements should be based on evidentiary facts, lest the expert opens themselves up to the possibility of an attack on cross examination.

In the same vein, hypothetical questions that go beyond the facts at issue and the purview of the case may cause a witness to riff and extrapolate their opinions without evidence. An expert may redirect the topic by stressing the questions present in the matter and how the evidence backs up their opinions.

The issue of whether an expert can offer an opinion on an ultimate issue of fact (or for that matter, what an ultimate issue of fact actually is) is not always clear-cut. The general principles of common law have held that an expert may not offer a legal conclusion—that is, an issue to be determined by the trier of fact. But at the same time, Rule 704 of the Federal Rules of Evidence expressly states that an expert opinion is “not objectionable just because it embraces an ultimate issue.” In other words, although an expert may opine on an issue of fact (i.e., the plaintiff’s injuries were caused by the vehicle’s high-speed collision), their testimony cannot state legal conclusions reserved for the jury to make (i.e., the negligence of the defendant’s driving proximately caused the plaintiff’s injuries). In order to avoid drawing legal conclusions when discussing issues of fact, an expert can be mindful of their choice of words. Does their wording reflect their own specialty and knowledge base or does it contain legalese? If the latter, the expert should reevaluate how they choose to set forth their opinions.

Overall, an expert’s credibility is a nearly immeasurable trait, with some possessing more innate credibility than others. But by avoiding certain phrasings, an expert may exponentially increase their chances of being trusted by a jury.

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