How to Cross Examine an Expert Witness

Expert witnesses are key witnesses in many types of lawsuits. For example, an expert typically testifies in a medical malpractice lawsuit or in a lawsuit claiming a product was defective. You must prepare for the cross-examination extensively. Research the expert's background and study their expert report inside and out. Generally, you can challenge an expert's testimony by challenging their credentials or the methodology and data they relied on.

Part 1

Part 1 of 3:

Analyzing the Expert

1
Read the expert report. An expert must provide a written report of their findings, and the other side's lawyer must give it to you ahead of time. You should read it several times. The expert report should form the basis of the expert's testimony in your case.
- If the expert relied on tests or data, that information should be provided as well (not simply the summary provided in the expert report). Request this information if necessary.
- In fact, request copies of any document the expert relied on when forming their expert opinion.^{[1]XResearch source} You'll want to check that the expert hasn't misread or misinterpreted data or other information.
- Not all expert witnesses have to submit a report. For instance, a physician can be both an expert witness and a "fact witness" in a personal injury case. Since they would be a kind of hybrid witness, a report is not required.
2
Study the expert's deposition testimony. The expert should also have testified in a deposition earlier in the lawsuit. In many ways, a deposition is a dry run for their trial testimony. Analyze the deposition testimony to see what the expert said.
- Remember you can use the expert's deposition testimony to impeach them during trial. For this reason, you want to know by heart what they said in the deposition.
- You can also use statements the expert made in depositions in other cases. Check the expert's CV for a full list of matters they have worked on. This CV should be provided to you.
- Find the deposition transcripts for these other cases. They might be publicly available in the courthouse file or you may need to contact an attorney who worked on the case.
- Read earlier deposition transcripts thoroughly. Sometimes, experts change their testimony from case to case.
Look for anything and everything. For example, an expert might have testified in a prior case. On appeal, the appellate court might have rejected the expert's testimony as unsound or illogical. You'll want to know that information in case the expert attempts to offer similar testimony in your case.
4
Hire your own expert. You'll need to present an expert witness at trial. Additionally, your own expert can help you identify weaknesses in the other side's expert witness. To find an expert witness, you should talk to another lawyer or contact an expert witness service.^{[2]XResearch source}
- Ask about fees. Experts generally charge by the hour. Remember that your expert will need to review material related to the case, write an expert witness report, and testify at trial and in a deposition. You also will want them to help you understand the issues. You should expect an qualified expert witness to be expensive.
- Make sure your own expert has impeccable credentials, as well as experience in being a witness, because they will be cross-examined by the other side's attorney.
- Ask your expert about the reputation of the expert you will cross-examine.^{[3]XResearch source}
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Part 2

Part 2 of 3:

Identifying Areas of Weakness

1
Analyze the expert's credentials. You can undermine an expert by challenging their expertise.^{[4]XTrustworthy SourceAmerican Bar AssociationLeading professional organization of lawyers and law studentsGo to source} This will require that you look at their education and experience, which should be listed in the expert report.
- Experts might inflate their expertise. Research whether they received the highest credential in their field. For example, an expert might testify as to medical issues when they never went to medical school.^{[5]XResearch source}
- An expert might have disciplinary actions taken against them. For example, a medical expert could have been sanctioned by their medical board.
- Remember that education and experience should relate to the issues they are testifying about. Someone might have a PhD in education but be attempting to testify as to legal malpractice issues. Their education isn't relevant.
- You might not know on your own whether the expert has sufficient expertise. If not, show the expert's qualifications to your own expert. Your expert can then assess whether the other side's expert has adequate credentials.
2
Identify holes in the expert's analysis. Even the most impressive expert witness report sometimes has holes in the analysis. If necessary, your own expert witness can identify them. You'll need to challenge them on cross-examination. For example, look for the following:^{[6]XTrustworthy SourceAmerican Bar AssociationLeading professional organization of lawyers and law studentsGo to source}
- Insufficient data. Check to make sure the expert had all the data necessary to offer a well-considered opinion.
- Errors. Sometimes experts make mistakes, especially when working with numbers. You need to double-check all mathematical equations and make sure they are accurate.
- Wrong assumptions. An expert might rely on research that shows how a certain drug affects people. However, all of the people in the study might have been middle-aged men of a certain weight. If your client is an underweight female, then the expert's evidence is not relevant.
Discuss with your own expert the methodology used by the expert you are cross-examining. Often, there are debates in the field about proper methodology. You will want to be able to ask questions about these disputes when you cross-examine the expert.
4
Determine if the expert is biased. You can severely hurt an expert's credibility by showing that they are biased. On cross-examination, you can ask questions that draw out the expert's bias, which can take many forms:
- Financial bias. Experts often get paid quite a lot of money to testify as experts. They may also expect additional work to come their way based on their testimony at trial. You can shine a spotlight on those financial rewards.^{[7]XResearch source} Dong so makes the witness appear less neutral in the eyes of the jury.
- Partisanship. You can ask the expert whether they always testify for the plaintiff or defendant. In this way, you can argue they are biased.
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Part 3

Part 3 of 3:

Conducting the Cross-Examination

1
Listen closely to direct testimony. Don't assume the expert will simply repeat what is in their expert report or what they stated at their deposition.^{[8]XTrustworthy SourceAmerican Bar AssociationLeading professional organization of lawyers and law studentsGo to source} If they change their testimony, you want to know that fact.
- You may need to change questions in your outline based on the direct testimony. Always be flexible.
- Remember to object if necessary. If the witness starts wandering, object to the narrative answer.
- Experts also can't offer an expert opinion as to the ultimate legal issue.^{[9]XResearch source} For example, an expert can testify that a child's behavior is consistent with child abuse. However, they can't opine that the defendant is guilty.
2
Draft leading questions. Leading questions can typically be answered with a “yes” or “no.” Although they are prohibited on direct examination, you can ask leading questions on cross-examination. Make sure to ask the expert leading questions since they give the expert fewer ways to deflect.^{[10]XTrustworthy SourceAmerican Bar AssociationLeading professional organization of lawyers and law studentsGo to source}
- You should have an outline prepared. Remember an outline is not a script.
- However, you should check your outline to make sure you have hit the points you intended to hit.
3
Ask questions about an expert's expertise. If you are challenging someone's expertise, then ask a series of questions that draw out why you think the expert is deficient in this area. Begin by getting the expert to confirm that they are qualified. Then draw out the gaps in their education or expertise. For example, you could ask the following set of questions (or something similar):^{[11]XResearch source}
- “Ma'am, you based your testimony on an analysis of the x-rays, correct?”
- “And you would agree that you have extensive experience in this field?”
- “Isn't it true that you've never worked as a radiologist?”
- “Or as a doctor?”
- “Or as a nurse?”
- “And isn't it true that your work experience has only been in occupational therapy, not internal medicine?”
4
Challenge the basis of the expert's opinion. You might want to highlight that an expert based their opinion on incomplete information. If so, you might want to ask a series of questions such as the following:
- “You didn't visit the scene of the crime, is that correct?”
- “Instead, you relied solely on the police report and other documents provided by the attorney?”
- “Did you see the defendant's car?”
- “Did you check the plaintiff's car?”
- “So your opinion that the brakes were faulty wasn't based on your personal observation, correct?”
- “Would you agree that your opinion is only as accurate as the information given to you?”
5
Expose bias. If you want to undermine the expert by showing bias, then ask a series of leading questions that show the expert had a reason to slant their testimony. For example, you could ask the following:
- “How often have you testified as an expert witness?”
- “Have you always testified for the plaintiff?”
- “How many times have you testified for a client of Mr. Smith?”
- “How much does Mr. Smith pay you an hour?”
- “What percent of your annual income consists of testifying for plaintiffs in medical malpractice cases?”
6
Impeach the expert with conflicting statements. A good way to undermine an expert witness' credibility is to confront them with a statement that conflicts with their testimony on direct examination. You can find conflicting statements in a variety of cases: the deposition, a deposition in a prior case, the expert report, or court testimony in a prior case.
- Make sure the conflicting statement is sufficiently serious. For example, it's probably pointless to challenge a doctor on the year they graduated medical school. They might say 1973 on the stand when their CV states 1974. People sometimes forget the exact date.
- However, some statements are serious. In a medical malpractice case, for example, the issue of whether a certain course of treatment was appropriate is an important issue.
- Experts might be prone to offering conflicting opinions when they are regular witnesses who represent both sides of an issue. For example, an expert might have testified in one case that a certain drug doesn't have certain side effects. If they get on the stand in your case and claim the opposite, you should confront them with the inconsistent statement.
7
Use a learned treatise to impeach the witness. It's acceptable under the rules of evidence to attempt to impeach a witness with a treatise. Essentially, a learned treatise is a text that is widely considered to be authoritative in the field. For example, a scholarly article might claim something contrary to what the expert has testified to. In order to impeach, you'll need to show the treatise is authoritative. You can do so in the following ways:^{[12]XResearch source}
- The witness acknowledges the treatise is authoritative. The witness can acknowledge that fact on the stand or in a deposition.
- Another expert acknowledges it is authoritative. For example, you could have your own expert testify to that effect.
- The judge exercises judicial notice. For example, a judge might accept The Reference Manual on Scientific Evidence as authoritative without needing any testimony.
8
Mitigate the damage caused by an effective witness. Some expert witnesses will be hard to cross-examine. They will have impeccable qualifications and offer well-reasoned testimony. You shouldn't bully an expert, since you will only look weak. Instead, come up with a way to salvage the cross-examination by having the expert endorse themes that are important to your case.
- For example, an expert might claim a surgical technique has caused the plaintiff to become paralyzed. You can try to get the expert to acknowledge that other doctors practice the same technique and that all surgery involves risk.
9
Remember your audience. Even the best juries might struggle to understand information that is very complex. You might have read an expert's report five times before understanding it. However, juries are confronting this information for the first time. Be mindful that you'll need to guide them through testimony.
- Ask the witness to explain complicated terms or concepts. Some concepts might be explained on direct, but new ones might be mentioned on cross.
- Don't overwhelm the jury. Identify the three or four most serious areas of weakness and concentrate on them. Any more and you risk boring the jury to tears.
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Tips

Don't overestimate the importance of expert testimony. Juries tend to be skeptical of experts to begin with.^{[13]XResearch source}
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You may also voir dire an expert, especially if you do not think they are sufficiently qualified to act as an expert. However, you might want to forgo voir dire for strategic reasons. For example, you might want to surprise the expert by waiting to challenge their expertise until you are in front of the jury. Note that "voir dire" is a procedure usually used to qualify the witness.
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