Assumptions are a nefarious tool of “hired gun” experts. If you do not critically look at the underlying assumptions of the expert, you could be allowing the expert to take unfair advantage of your client’s case. Assumptions are the foundation upon which an expert opinion rests. A house with an unsound foundation lacks structural integrity and is potentially dangerous to the occupants. The same is true for experts. You can hit them where they live, if you can show their assumptions are unsupported by the evidence or science. Your starting point is identifying what assumptions the expert is making. Is there a more accurate source for this information?

Guesses Versus Actual Evidence
For example, an expert in determining speed uses a table for the coefficient of friction for a roadway based upon the type of surface it has, asphalt. He is assuming that all black top roadways have the same coefficient of friction. However, this varies from roadway to roadway. The most accurate way to find the coefficient of friction is to actually measure it. Textbooks on accident reconstruction in fact recommend that you do this if possible. Or an expert uses photographs to find specific locations for purposes of scene measurements. However, instead of using the more time-consuming, exact and accepted method of triangulation, he “assumes” that he can just “eyeball” the site. The expert may fail to find the type of camera and len’s length that was used to take the scene photographs. If a zoom lens is used, it can lead to visual distortions of what is being depicted. Everyone for example has seen a picture of what appears to be a “giant” moon when the camera shot is taken with a telephoto lens.

Assumptions About Credibility
Is the expert choosing between two conflicting pieces of evidence and selecting the evidence that best supports his client? If two eye-witnesses have testified to different versions of what occurred, the jury, not the expert, has to assess the witnesses’ credibility and weigh the evidence in order to determine the outcome.

Garbage in Garbage Out
Has the expert missed or ignored critical evidence that could affect his determination of causation or etiology? When a doctor makes a differential diagnosis, he has to eliminate other conditions or illnesses that could give rise to the same symptoms. For example, a person is claiming permanent brain damage from a collision. However, the same person has a long history of alcohol abuse and atrophy of the brain. This condition can lead to the same sort of cognitive problems/symptoms as a traumatic brain injury per the Diagnostic and Statistical Manual of Mental Diagnosis. However, the expert is unaware of the condition. Per the DSM-V this history is listed as a condition which must be eliminated for an correct differential diagnosis to be made. The doctor or neuro-psychologist has assumed that the person has a normal brain prior to the collision. This faulty assumption rocks the very foundation of the expert’s opinion and undermines his credibility before the jury. Most expert’s will agree that in order to reach accurate opinions, you must first have accurate and complete information.

Real Science versus Junk Science
Has the expert embarked upon and reached an opinion through means or methods which are not accepted in the scientific community at large? Is there main stream science which supports their means of reaching such a conclusion? Has their method of analysis been shown forensically to reach correct determinations. If the hired gun did a paper review of a file and determined that your client was not injured or will not have medical problems in the future, has the expert ever conducted a study in his cases to see how accurate his opinions are? Has a scientific journal found or study ever been performed validating medical records reviews as accurate predictors? Does the doctor or expert agree to accept professional responsibility if his conclusion is incorrect and your client is denied future medical benefits or can no longer work due to their injury? In other words, is the expert willing to put his money where his mouth is?

Does the method meet the requirements under Rule of Evidence 702 and the Frye Test or Daubert Analysis? See Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579, 113 S. Ct. 2786, 125 L. Ed. 2d 469, (1993). The Daubert rule states, “If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.” In Daubert, the Court stated that evidence based on innovative or unusual scientific knowledge may be admitted only after it has been established that the evidence is reliable and scientifically valid. The older Frye test announced in Frye v. United States, 293 F. 1013 (D.C. Cir. 1923), is met if the evidence sufficiently established that the method has gained general acceptance in the particular field in which it belongs.

Is the evidence relied upon by the opposing expert, the sort of evidence generally relied upon by experts in the field to make a determination as required under Rule of Evidence 703. This rule provides that:

An expert may base an opinion on facts or data in the case that the expert has been made aware of or personally observed. If experts in the particular field would reasonably rely on those kinds of facts or data in forming an opinion on the subject, they need not be admissible for the opinion to be admitted. But if the facts or data would otherwise be inadmissible, the proponent of the opinion may disclose them to the jury only if their probative value in helping the jury evaluate the opinion substantially outweighs their prejudicial effect.

Assuming Expertise in an Area Outside Their Field of Expertise
Is the expert venturing outside of his field? Is he really an expert on the matter he is testifying about? Void dire the expert on his qualifications and have the court exclude him if possible, or narrow his expertise and render his opinion useless or unreliable. Just because you are an expert in one area, does not make you an expert in all areas.

Arguing the Unfairness of Assumptions to the Jury

A good argument I have heard made on the unfair use of assumptions goes something like this:

When you are deliberating over your verdict, and thinking about and weighing the evidence in this case, ask yourself whether or not you are making assumptions. Ask yourself whether you are allowing opposing counsel or his so called expert to put their thumb on the scales of justice. This is something you must carefully guard against – you must make sure this “thumb” plays no part in your deliberations or in your verdict.

Remember, when you assume, you make an “ass” out of “you” and “me.”

I hope these thoughts help you spot faulty assumptions being made by experts and their weaknesses so that you can take advantage of the opportunities they present on cross-examination.