PastPaperHero | Relevancy and reasons for excluding relevant evidence

Learning Outcomes

This article explains expert testimony in the context of relevancy and exclusion of relevant evidence, including:

Identifying when expert testimony is relevant, genuinely helpful to the trier of fact, and required to resolve specialized issues on the MBE.
Explaining the threshold admissibility requirements under Rule 702, focusing on expert qualification, sufficient factual basis, and reliable principles and methods.
Applying the Daubert reliability standard and its flexible factors to scientific, technical, and experience-based testimony across common exam fact patterns.
Distinguishing clearly between lay opinion under Rule 701 and expert opinion under Rule 702, including tricky “dual-role” witnesses like police officers and treating physicians.
Recognizing and evaluating expert qualification issues, including when limited training or experience is sufficient and when it only affects weight, not admissibility.
Determining when otherwise relevant expert testimony must be excluded or limited under Rule 403 because of unfair prejudice, confusion, or needless cumulativeness.
Analyzing how experts may base opinions on inadmissible facts or data under Rule 703 and when those underlying materials may be disclosed to the jury.
Assessing limitations on expert testimony about ultimate issues and criminal mental states under Rule 704, especially in insanity and self-defense questions.
Strategically approaching MBE-style questions involving expert testimony by working through relevance, reliability, Rule 403, and ultimate-issue constraints in a structured order.

MBE Syllabus

For the MBE, you are required to understand the rules governing expert testimony and the exclusion of otherwise relevant evidence, with a focus on the following syllabus points:

Recognizing when expert testimony is relevant and helpful to the trier of fact.
Identifying the threshold requirements for expert opinion evidence (Rule 702).
Applying the Daubert reliability framework, including its non-exclusive factors.
Distinguishing between lay and expert opinion under Rules 701 and 702.
Understanding reasons for excluding relevant expert evidence under Rule 403 (unfair prejudice, confusion, waste of time, etc.).
Knowing the judge’s gatekeeping role and how preliminary admissibility determinations are made (Rule 104).
Understanding the bases on which experts may form opinions (Rule 703) and how those bases may be presented to the jury.
Recognizing limits on expert testimony, including ultimate issue and credibility restrictions.

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

Which of the following is NOT a required element for admissibility of expert testimony?
1. The expert is qualified by knowledge, skill, experience, training, or education.
2. The expert’s opinion is based on sufficient facts or data.
3. The expert’s opinion is based solely on personal belief.
4. The expert’s methods are reliably applied to the facts.
Under the Daubert standard, which factor is LEAST relevant to determining the reliability of expert testimony?
1. Whether the theory has been tested.
2. Whether the theory is generally accepted in the field.
3. Whether the expert is a friend of the judge.
4. Whether the error rate is known.
A relevant expert opinion may be excluded if:
1. It is based on reliable methods.
2. Its probative value is substantially outweighed by the danger of unfair prejudice.
3. The expert is qualified.
4. The testimony is helpful to the jury.

Introduction

Expert testimony often determines the outcome of cases involving scientific, technical, or other specialized knowledge—medical malpractice, defective products, forensic analysis, engineering, and many more. But on the MBE (and under the Federal Rules of Evidence), it is not enough that expert evidence is simply relevant. The proponent must satisfy specific admissibility requirements, and even relevant, technically admissible expert testimony can still be excluded.

At a high level, think of three separate questions:

Is the testimony relevant and helpful to the trier of fact?
Is it reliable and properly founded (expert qualification, sufficient data, reliable methods)?
Should it nevertheless be excluded because of unfair prejudice, confusion, or other Rule 403 concerns?

Understanding how these layers interact is central to handling expert testimony questions on the MBE.

Key Term: Expert Testimony
Opinion evidence given by a witness qualified as an expert, based on specialized knowledge, skill, experience, training, or education, that assists the trier of fact in understanding evidence or determining a fact in issue.

Key Term: Expert Qualification
The combination of a witness’s specialized knowledge, skill, experience, training, or education that is sufficient to permit the witness to give expert opinion on a particular subject under Rule 702.

Relevance and the Role of Expert Testimony

Expert testimony is relevant when it has any tendency to make a fact of consequence more or less probable and, more specifically under Rule 702, when it will help the trier of fact understand the evidence or determine a fact in issue.

Two ideas are embedded in the “helpfulness” requirement:

The topic must involve scientific, technical, or other specialized knowledge not ordinarily within the understanding of lay jurors.
The testimony must actually aid the jury rather than simply tell them what result to reach.

Classic examples where experts are commonly used:

Medical causation (e.g., whether a drug can cause a particular injury).
Accident reconstruction (e.g., speed and angles in a collision).
Product design and defects (e.g., whether a design is unreasonably dangerous).
Forensic analysis (e.g., cause of death, ballistics, DNA interpretation).
Professional standards of care (e.g., medical or legal malpractice).

Conversely, expert testimony is generally unnecessary and may be excluded if:

The subject is within ordinary experience (e.g., whether a sidewalk was slippery, whether a person seemed drunk).
It would simply restate what jurors can infer for themselves.

Courts are also cautious about expert testimony on witness credibility—e.g., “In my opinion, this child is telling the truth.” Such opinions are usually excluded as unhelpful and as invading the jury’s role.

Threshold Requirements for Expert Testimony

The Federal Rules (and the MBE) follow Rule 702. For expert testimony to be admissible, five threshold requirements must be satisfied:

The witness is qualified as an expert.
The testimony will help the trier of fact.
The opinion is based on sufficient facts or data.
The opinion is the product of reliable principles and methods.
The expert has reliably applied those principles and methods to the facts.

Each element is a potential testing point.

1. Expert is qualified

The witness must have specialized knowledge, skill, experience, training, or education.

Formal degrees or licenses are helpful but not required.
Practical experience (e.g., a long-time mechanic, accident investigator, or nurse) can be enough.
The qualification must match the subject of testimony: a neurosurgeon may not be qualified to testify about structural engineering, and an engineer may not be qualified to give causation opinions in oncology.

Challenges to expert qualification are common on cross-examination and go to both admissibility and weight.

Key Term: Gatekeeping Role
The trial judge’s responsibility under Rules 104 and 702 to determine, before the jury hears it, whether proposed expert testimony is sufficiently qualified, relevant, and reliable to be admitted as evidence.

2. Testimony will help the trier of fact

The expert’s specialized knowledge must help the jury or judge understand evidence or determine a fact in issue. If jurors can understand the issues without an expert, the court may exclude the testimony as unnecessary.

For the MBE, when you see:

Complex technical or medical issues → expert testimony is likely helpful.
Simple, everyday observations → expert testimony may be unnecessary or excluded.

3. Opinion based on sufficient facts or data

The expert must have an adequate factual basis, which may come from:

Personal observation or examination (e.g., physically examining a patient).
Evidence presented at trial (e.g., listening to testimony, reviewing admitted exhibits).
Information supplied outside the courtroom that other experts would reasonably rely on (e.g., lab reports, medical records, police reports).

A single anecdote or case report, or speculation without real data, is usually inadequate.

4. Reliable principles and methods

The opinion must be grounded in methods that are reliable in the relevant field. This is where the Daubert standard applies.

Key Term: Daubert Standard
A framework used to assess the reliability and relevance of expert testimony, considering factors such as testability, peer review, error rate, standards, and general acceptance in the relevant field.

5. Reliable application to the case

Even reliable methods can be misapplied. The expert must apply the methodology in a way that is reasonable and fits the facts of the case.

Example: A reliable statistical model applied to data outside its validated range may not be reliably applied.
The MBE may test whether the expert’s method is sound in theory but poorly matched to the specific facts.

The Daubert Reliability Standard

The trial judge acts as a gatekeeper, screening out unreliable expert evidence before it reaches the jury.

Under Daubert (and Kumho Tire), courts consider a non-exclusive list of factors:

Can the theory or technique be tested, and has it been tested?
Has it been subjected to peer review and publication?
What is the known or potential error rate?
Are there standards that control the technique’s operation, and are they maintained?
Is the method generally accepted in the relevant scientific or technical community?

These factors are flexible; not every factor must be satisfied, and courts may consider other indicators of reliability. The key MBE points:

Formal scientific methods and more experience-based technical methods (e.g., tire failure analysis) are both subject to Daubert review.
Traditional general acceptance (the old Frye test) is no longer the exclusive measure; it is just one factor.

Reasons for Excluding Relevant Expert Evidence

Even when expert testimony is relevant and satisfies Rule 702, it may still be excluded under Rule 403.

Key Term: Rule 403 Exclusion
The court’s authority to exclude relevant evidence if its probative value is substantially outweighed by dangers such as unfair prejudice, confusing the issues, misleading the jury, undue delay, waste of time, or needless cumulative evidence.

Common Rule 403 concerns with expert testimony:

Unfair prejudice: For example, a highly credentialed expert with a speculative opinion might unduly sway the jury.
Confusing the issues or misleading the jury: Highly complex technical testimony may distract from core issues or encourage the jury to give excessive weight to shaky science.
Waste of time / cumulative evidence: A parade of experts repeating the same point can be excluded as needlessly cumulative.

On the MBE, be alert that:

Even if an expert is qualified and the methods are reliable, Rule 403 can still justify exclusion.
The standard is high: the danger must substantially outweigh the probative value.

Lay Opinion vs. Expert Opinion

The MBE frequently tests the boundary between lay and expert opinion.

Key Term: Lay Opinion
Testimony by a non-expert witness that is limited to opinions rationally based on the witness’s perception, helpful to understanding the testimony or determining a fact in issue, and not based on scientific, technical, or specialized knowledge.

Lay opinion is admissible when:

It is rationally based on the witness’s own perception (Rule 701).
It is helpful to understanding the testimony or a fact in issue.
It is not based on specialized, scientific, or technical knowledge.

Examples of admissible lay opinions:

“He looked drunk.”
“The car was going very fast.”
“She seemed angry.”
A non-expert identifying a voice or handwriting with which they are familiar.

If the witness’s opinion relies on specialized training or experience—such as interpreting drug slang based on years of narcotics investigations—it typically becomes expert testimony and must satisfy Rule 702.

Expert opinion, by contrast, can:

Address matters beyond common experience.
Rely on data and methods not accessible to lay witnesses.
Be based on facts not personally perceived if experts in the field reasonably rely on them.

A frequent trap is a police officer or professional giving testimony that straddles the line between lay and expert. If the opinion could be formed by any layperson based on observation (“The car seemed to be speeding”), it is lay opinion. If it rests on specialized knowledge (“Based on skid mark analysis, the vehicle was traveling 72 mph”), it is expert testimony.

Subject Matter and Ultimate Issues

Expert testimony is allowed on most subjects where specialized knowledge will assist the trier of fact.

However, there are limits:

Courts generally reject expert testimony on witness credibility (e.g., “Children rarely lie about abuse”) because it usurps the jury’s role.
Experts may give opinions on the ultimate issue in most civil and criminal cases.

Key Term: Ultimate Issue Opinion
An expert’s opinion that goes directly to a fact the jury must decide (e.g., causation, defect, standard of care), which is generally permissible under Rule 704, except for opinions on a criminal defendant’s mental state that constitutes an element of the crime or defense.

Under Rule 704(a), an opinion is not objectionable merely because it embraces an ultimate issue. Examples:

“The brake design was unreasonably dangerous.”
“The plaintiff’s injuries were caused by the accident.”
“The patient’s treatment fell below the standard of care.”

Exception (Rule 704(b)): In criminal cases, an expert may not state an opinion about whether the defendant did or did not have the mental state or condition constituting an element of the crime or a defense (e.g., “The defendant intended to kill the victim,” “The defendant knew right from wrong”). The expert may describe the defendant’s mental condition, diagnoses, and symptoms, but not the ultimate legal mental state.

Basis of Expert Opinions (Rule 703)

Key Term: Basis of Expert Opinion
The facts or data on which an expert relies, which may include personal observations, evidence presented at trial, and information reasonably relied upon by experts in the field, even if that information itself is inadmissible.

An expert may base an opinion on:

Personal knowledge: Examination of a patient, inspection of a product, viewing an accident scene.
Evidence presented at trial: Listening to witnesses or reviewing exhibits.
Facts or data outside the courtroom: Reports, tests, and other information not in evidence, if experts in the field reasonably rely on such materials.

Important MBE points:

The facts or data themselves may be inadmissible (e.g., hearsay) but can still be used to form the expert’s opinion.
The opinion may be admitted even if the supporting facts/data are not.

However, Rule 703 places limits on disclosing supporting inadmissible facts to the jury:

Supporting inadmissible facts may be disclosed to the jury only if their probative value in helping the jury evaluate the opinion substantially outweighs their prejudicial effect.
This is essentially a reverse Rule 403 balancing for the basis of the opinion.

Learned Treatises

Key Term: Learned Treatise
A published work (such as a treatise, periodical, or manual) recognized as a reliable authority in a field, which may be used with an expert witness and read into evidence under the learned treatise hearsay exception.

Learned treatises can:

Be used on direct examination to show the basis of an expert’s opinion.
Be used on cross-examination to challenge or support an expert’s conclusions.

The text of the treatise is read to the jury but is not received as a physical exhibit. This interacts with expert testimony because treatises often serve as part of the expert’s factual basis and as tools for impeachment.

Judge’s Gatekeeping Role

The trial judge decides, as a preliminary matter, whether expert testimony is admissible.

Under Rule 104(a), the judge determines preliminary facts that affect admissibility, such as:
- Whether the witness is qualified as an expert.
- Whether the methods are reliable under Daubert.
- Whether the expert has sufficient data and applied the methods reliably.

The judge:

May consider evidence that would not itself be admissible at trial when resolving these preliminary questions.
May hold a Daubert hearing outside the presence of the jury.
May appoint a court-appointed expert in appropriate cases, who can be deposed and examined by both sides.

This gatekeeping function is particularly important for expert testimony because jurors might give excessive weight to “scientific” evidence; the court must ensure that what reaches them is both relevant and reliable.

Worked Example 1.1

A plaintiff sues a car manufacturer, alleging a design defect caused a crash. The plaintiff offers an engineer as an expert. The engineer’s opinion is based on a method not generally accepted in automotive engineering, has never been peer-reviewed, and has an unknown error rate. The manufacturer objects.

Answer:
The judge should exclude the expert’s opinion. Under the Daubert standard, the method’s lack of general acceptance, absence of peer review, and unknown error rate all weigh strongly against reliability. Because the judge serves as gatekeeper for reliability under Rule 702, the court should rule that the expert’s methodology is not sufficiently reliable to be presented to the jury. The opinion is therefore inadmissible, even if the expert is otherwise well qualified.

Worked Example 1.2

A medical expert is called to testify that a certain drug caused the plaintiff’s injury. The expert is qualified and the testimony is relevant, but the expert’s opinion is based on a single case report and personal belief, not on scientific studies or accepted methodology.

Answer:
The court should exclude the testimony. Although the expert is qualified and the issue is relevant, the opinion is not based on sufficient facts or data and does not appear to be the product of reliable principles and methods. A single case report plus personal belief falls short of the Daubert reliability standard. On the MBE, an opinion based solely on anecdote or speculation is inadmissible expert testimony.

Worked Example 1.3

In a criminal arson trial, the prosecution calls a fire investigator as an expert to testify that, in his opinion, the fire was intentionally set. The investigator is experienced and uses standard fire investigation techniques. He testifies that he reviewed police reports containing eyewitness statements that would be inadmissible hearsay if offered directly. The defense objects that the opinion is based on hearsay and must be excluded.

Answer:
The court should admit the expert’s opinion. Under Rule 703, an expert may base an opinion on facts or data that are not themselves admissible, so long as they are of a type reasonably relied upon by experts in the field. Fire investigators reasonably rely on reports from other responders, including police reports and witness statements. The hearsay nature of the reports does not bar the expert’s opinion. However, the supporting hearsay statements are not automatically disclosed to the jury; they may be elicited only if their probative value in helping the jury evaluate the opinion substantially outweighs any prejudicial effect.

Worked Example 1.4

In a negligence case arising from a car accident, a lay witness testifies, “The car was going at least 75 miles per hour.” The defendant objects that the witness is not an expert in accident reconstruction and cannot testify in terms of miles per hour.

Answer:
The objection should be overruled. This is lay opinion testimony, not expert testimony. Under Rule 701, a lay witness may give opinion testimony if it is rationally based on perception and helpful to the jury. Estimating speed of a moving vehicle based on observation is within ordinary experience and does not require scientific or technical specialized knowledge. Therefore, the witness’s opinion is admissible without satisfying the expert requirements of Rule 702.

Worked Example 1.5

A psychologist is called by the defense in a murder trial to testify that, at the time of the killing, the defendant believed she had to kill the victim in order to save her own life and therefore acted in self-defense. The prosecution objects.

Answer:
The objection should be sustained. While a psychologist may testify about diagnoses, mental conditions, or the effects of trauma, Rule 704(b) prohibits expert testimony in a criminal case that states an opinion about whether the defendant had the mental state or condition constituting an element of the crime or a defense. Saying that the defendant “believed she had to kill in self-defense” effectively states that she possessed the mental state required for the self-defense defense, which is barred as an ultimate-issue mental-state opinion.

Exam Warning

The MBE may test whether expert evidence is excludable even if it is relevant and the expert is qualified. Always:

Check that the expert’s methodology and application satisfy Daubert.
Analyze whether the opinion is based on sufficient facts or data.
Consider Rule 403: complex or dramatic expert testimony may be excluded if its probative value is substantially outweighed by unfair prejudice, confusion, or waste of time.
In criminal cases, remember the mental state limitation on ultimate-issue expert opinions.

Revision Tip

When answering MBE questions on expert testimony, proceed in order:

Is the subject matter appropriate for expert testimony (specialized knowledge; helpfulness)?
Is the witness qualified in that specialty?
Is the opinion grounded in sufficient facts or data and reliable methods (Daubert factors)?
Has the expert applied those methods reliably to the facts?
Does any Rule 403 problem justify exclusion despite relevance and reliability?
In criminal cases, make sure the expert is not opining directly on the defendant’s mental state as an element of the offense or defense.

Key Point Checklist

This article has covered the following key knowledge points:

Expert testimony must involve specialized knowledge and be helpful to the trier of fact.
The expert must be properly qualified in the relevant field.
Rule 702 requires sufficient facts or data, reliable principles and methods, and reliable application to the case.
The Daubert standard provides flexible factors for assessing reliability; general acceptance is only one factor.
The judge acts as a gatekeeper, deciding admissibility before the jury hears expert evidence.
Experts may base opinions on personal observation, trial evidence, and reasonably relied-upon outside data, even if that data is inadmissible.
Supporting inadmissible data may be disclosed to the jury only if its probative value substantially outweighs its prejudicial effect (Rule 703).
Experts may testify to ultimate issues except they may not opine directly on a criminal defendant’s mental state as an element of the crime or defense.
Relevant expert evidence can still be excluded under Rule 403 if its probative value is substantially outweighed by unfair prejudice, confusion, or waste of time.
Lay opinion is limited to perceptions, must be helpful, and cannot be based on scientific, technical, or specialized knowledge.
Expert opinions must be grounded in more than mere personal belief or anecdote and cannot simply vouch for witness credibility.

Key Terms and Concepts

Expert Testimony
Expert Qualification
Daubert Standard
Rule 403 Exclusion
Lay Opinion
Gatekeeping Role
Ultimate Issue Opinion
Basis of Expert Opinion
Learned Treatise

Relevancy and reasons for excluding relevant evidence - Expe...