Car accident cases use expert opinions witnesses in several respects. An accident reconstruction expert may render an expert opinion on how the accident happened and in effect what caused the crash. The attorneys use this expert opinion to argue who is at fault. A medical doctor expert may give an opinion of several issues: namely, what injuries were caused by the collision, what treatment was reasonable and necessary for those injuries, is there a permanent injury and what future medical is needed to treat those accident related injuries. Finally, a vocational and/or economist expert may testify as to past and future loss of earning capacity. There could also be other types of experts needed depending on the specific facts of each case.
Wisconsin Civil Jury Instruction 260 explains expert opinions:
Usually, witnesses can testify only to facts they know. But, a witness with expertise in a calling or specialty may give an opinion in that calling or specialty. In determining the weight to be given an opinion, you should consider the qualifications and credibility of the expert and whether reasons for the opinion are based on facts in the case. Opinion evidence was admitted in this case to help you reach a conclusion. You are not bound by any expert's opinion. In resolving conflicts in expert testimony, weigh the different expert opinions against each other and consider the qualifications and credibility of the experts and the reasons and facts supporting their opinions.
Sometimes, the attorneys submit scholarly articles which support their case and Wisconsin Civil Jury Instruction 261 on medical or scientific treatise in evidence explains:
During the trial, the court received portions of a published treatise in evidence written by an expert. This evidence was not received to establish that the views expressed by the author are undisputed truths or absolute standards. The author's views should be considered as any other expert witness' opinion. You are not bound by the opinions expressed by the author any more than you are bound by the opinion of any other expert witness.
The attorneys may ask experts hypothetical questions which assume basic disputed facts and Wisconsin Civil Jury Instruction 265 shows how:
During the trial, an expert witness was told to assume certain facts and asked for an opinion based upon the assumed facts. This is called a hypothetical question. Consider the opinion in answer to the question only if you believe the assumed facts upon which it is based. If you find that the assumed facts in the hypothetical question have not been proved, do not give any weight to the opinion.
McCormick Law Office in Milwaukee, Wisconsin uses car accident expert opinions in every case as necessary and best.