22: Being an expert


Today is the last new content day of the course. We'll be doing wrap-up and review on Tuesday.

Please fill out the online SRTIs! I really do read, appreciate, and try to use all the feedback that students give me.


Remember earlier in the semester we talked about being an expert witness. The judge is the ultimate arbiter, and there are various rules that must be followed to establish an expert's bona fides.

A dramatic example

Amusingly, there is a studio movie from 1992 that is still used in law school to demonstrate various aspects of this process (and how to effectively utilize an expert witness): My Cousin Vinny.

We are not going to watch the entire movie (though you might: it's pretty good). Instead we're going to watch a short excerpt to get a sense of the relevant bits.

Some setup: Billy and Stan are charged with murder due to a series of misunderstandings: a murder did happen, and they were there, but the movie makes clear they are not the murders (it's a comedy of errors sort of thing).

Billy's cousin Vinny is sent from New York to be his defense attorney. Vinny has no courtroom experience, but he is a cunning fellow. The case is going not terribly well (for Billy).

Vinny does manage to show some of the prosecutor's eyewitness's testimony is not valid...


So the prosecutor plays their last (best?) card.


The prosecutor's ace in the hole is an expert witness from the FBI, an expert on tire track identification.

There are reasonable procedural objections to the ace in the whole (which Vinny raises thanks to earlier Mona Lisa educational intervention (she's his fiancee)) but the Judge overrules.

The expert is well qualified and testifies convincingly that the rubber left on the scene are of the same chemical type of rubber as on Billy's car, and that the tracks that were left were from the same type of tire as on Billy's car.


Vinny does a "constructive cross examination" on the prosecution's expert witness, seeking his agreement on fundamental facts. The idea here is that if the prosecution and the defense both question the expert, and he agrees to something, then it's indisputed.

Without ever disputing a fact, Vinny elicits the admission that the tires in question are the most popular tires used on cars that year. This implicitly weakens the implication that the car belonged to the defendants, since there are hundreds of thousands of other cars that might have had the same tires.

(This is a courteous cross, and one that doesn't imply the expert is corrupt or incompetent. The prosecution did not ask their expert to rule out alternatives, but only to buttress their assertion that Billy was the murderer, despite the impeached testimony of the eyewitnesses. This highlights a flaw with )


Vinny (a car buff) realizes there is something afoot, but he can't testify himself.

He goes to fetch Mona Lisa as his expert, since he knows she's also a car buff (and a mechanic).

Due to him being a dick, she's pissed at him right now so she comes reluctantly at best. Legally this is interesting! She's strongly biased (for, since they're engaged: against, since she's pissed, possibly in other ways) and serves to illustrate bias in general (e.g., is the expert paid?). Normally attorneys (and experts) must be prepared to refute accusations of bias, but Vinny subverts that here (for comic effect, thanks to Hollywood).


He then baits an error out of the prosecution, who is convinced ladies don't know anything about cars. He proposes an open-court voir dire (usually held with just the judge). The prosecution attempts to show she's unqualified. In open court, voir dire is essentially a mini-cross exam, aimed to show the legal sufficienty of an expert's qualifications, by knowledge, skill, experience, training, or education.

The prosecutor immediately challenges and attempts to show she's unqualified.


She refutes, first by family. Then by her own work. But notably, Vinny introduced her as an expert on general automotive knowledge, not tire tracks! This will make her testimony (assuming it's relevant to general knowledge) more powerful than the tire track expert, assuming that it's not solely related to tire tracks. But Vinny needs to establish her as an expert.


"She's acceptable, your honor."

Context matters here. Mona doesn't have a PhD in automotive mechanics and she doesn't have to. This is a rural town where everyone owns one or more vehicles -- cars are an important part of their lives. Judge and jury all know who the good mechanics are, and likely they are family mechanics. To claim familial expertise in this context is as natural as claiming academic expertise in more intellectual pursuits.

Her recognition that the question is a trick, and immediate careful, detailed, and clear explanation shifts the balance of control to her. By taking the time to understand the question (which in her case was not much!) she eliminates doubt that should is an expert in the area in question. It will also potentially allow Vinny to argue that the government expert is not an expert on the relevant information: tire tracks alone aren't determinative here, as we'll see.

Finally she is directly examined, which usually wouldn't happen in real life. But dramatic license and all that. Normally the expert examines evidence in advance, prepares a report for both prosecution and defense, then testifies about it (also under cross).

Here, because drama, the government expert was a surprise, so Mona is too.

Mona examines the picture and comes to the conclusion it exonerates Billy.


Car is different: Positraction vs regular differential, and solid axle vs independent rear suspension. Only two cars made in the 60s that met these conditions could make the tracks: a Corvette (no way) and a '63 Tempest, very similar to the boys' Skylark.


Vinny then recalls the government expert and uses him to corroborate Mona's testimony. Note that he's not impugning the government witness directly.

Shakespearean-level drama aside, MCV is a well-cited demonstration of how two honest and qualified experts can give conflicting testimony on an issue. It's not unusual for an attorney and/or expert to fail to consider a fact or hypothesis, nor is it unusual for an attorney to request a review of only a subset of facts or hypotheses from an expert.

It's important for experts to consider all facts unless they are unconcerned about being placed in the same position as the FBI's expert in MCV. And they typically need more time than this dramatic staging to review and prepare for their testimony. Notably (as hinted at by the overruled objection in MVC), the rules of evidence require that parties disclose both evidence and expert findings before the trial, so that both sides can adequately prepare their cases.

A real-world example

Consider the following affadavit from a real forensic expert in a real trial.

This is an affadavit in a case related to copyright infringment using p2p networks.

The prosecution expert (not shown) claims that an IP log suffices to identify an individual distributing files in violation of copyright.

The defense expert (here) disputes this assertion.

It starts with an list of qualifications (1—-15), some formal and some experiential.

16 discloses the witness's interest (he's a retained expert).

17--18 list the evidence that was considered. 19 lists external (accepted in the field) references.

20 states he's reviewed the prosecution's expert's testimony and that he disagrees.

21-23 explain why the simple one-ip-per-computer explanation of the p.e. is wrong.

24--25 explain why an ISP might not know exactly which computer(s) is/are assigned which IP address.

26--27 claims that individuals (as opposed to IP addresses) are not uniquely identified.

28 parses the point specifically, noting again that an IP address is not necessarily an identifier of a person. NATs aside, there are other hypotheses that raise doubts.