Argument Visualization Tools for Corroborative Evidence

2nd International Conference on Evidence Law and Forensic Science (ICELFS 2009)Beijing, China, July 25, 2009Douglas Walton (CRRAR)University of Windsorhttp://www.dougwalton.ca

Defining Corroborative Evidence

Corroborative evidence can be broadly defined as any evidence that further supports some evidence that already exists in a case.

The evidence that is already there can be called the primary evidence, and the evidence that supports it can be called the secondary evidence.

What is meant when it is said that the secondary evidence supports the primary evidence is that the secondary evidence increases the probative weight of the primary evidence.

An Example

A witness testified that she saw the defendant drive his car into a red car. Subsequent to that a second witness testified that he saw red paint on the fender of the defendant’s car on the day after the accident.

In this case both kinds of evidence are based on testimony.

The primary evidence is the testimony of the first witness that she saw the defendant drive his car into the red car.

The secondary, or corroborating evidence, is the testimony of the second witness that he saw red paint on the fender of the defendant's car.

Questioning the Definition

Some would say that the red paint example is not really a case [strictly speaking] of corroborative evidence.

Reason: it may not be true that the red paint evidence increases the probative weight of the witness testimony evidence.

Still, if both pieces of evidence support the conclusion that the defendant drove his car into the red car, the case is [in a broader sense] one of corroborative evidence.

Two Kinds of Arguments

In a linked argument, the premises function together as a reason to support the conclusion.

Example: Bob is an expert on paint matching; Bob says that this paint sample matches that one; therefore this paint sample matches that one.

In a convergent argument, each premise is a separate reason for the conclusion.

Example: A witness testified that she saw the defendant drive his car into a red car. Subsequent to that a second witness testified that he saw red paint on the fender of the defendant’s car on the day after the accident.

Example of a Linked Argument

Bob is an expert on paint matching.

This paint sample matches that one.

Bob says that this paint sample matches that one.

Example of a Convergent Argument

A witness testified that she saw the defendant drive his car into a red car.

The defendant drove his car into a red car.

A second witness testified that he saw red paint on the fender of the defendant’s car on the day after the accident.

A Question about Independence

In the red paint example [classified as a convergent argument] is each premise an independent reason?

Maybe the paint matching evidence increases the probative weight of the eyewitness testimony evidence.

This would mean that the one reason is not independent from the other.

Argument from Expert Opinion

Major Premise: Source E is an expert in subject domain S containing proposition A.

Minor Premise: E asserts that proposition A is true (false). Conclusion: A is true (false).

Critical Questions for Argument from Expert Opinion

CQ1: Expertise Question. How credible is E as an expert source? CQ2: Field Question. Is E an expert in the field that A is in? CQ3:Opinion Question. What did E assert that implies A? CQ4: Trustworthiness Question. Is E personally reliable as a source? CQ5: Consistency Question. Is A consistent with what other experts

assert? CQ6: Backup Evidence Question. Is E's assertion based on

evidence?

Argument from Witness Testimony

Position to Know Premise: Witness W is in position to know whether A is true or not.

Truth Telling Premise: Witness W is telling the truth (as W knows it). Statement Premise: Witness W states that A is true (false). Conclusion: A may be plausibly taken to be true (false).

Critical Questions for Argument from Witness Testimony

CQ1: Is what the witness said internally consistent? CQ2: Is what the witness said consistent with the known facts of the

case (based on evidence apart from what the witness testified to)? CQ3: Is what the witness said consistent with what other witnesses

have (independently) testified to? CQ4: Is there a bias that can be attributed to the witness? CQ5: How plausible is the statement A asserted by the witness?

Three Ways One Argument Can Support Another Argument

Example of Argument Support

Araucaria

Araucaria is a free software tool for analyzing arguments that helps a user to diagram a given argument using a point-and-click interface.

The user moves the text of discourse into a box on a left window of the Araucaria interface, and then highlights each statement (premise or conclusion).

The user can then draw an arrow representing each inference from a set of premises to a conclusion.

Araucaria was the first argument visualization tool to incorporate the use of argumentation schemes.

Araucaria Menu with Critical Questions

Corroborative Expert Opinion Evidence in Araucaria

Corroborative Expert Opinion Evidence in Rationale

because because because because

P is true.

Source 1 is an expert. Source 1 says that P.

Expert Opinion

Source 2 is an expert. Source 2 says that P.

Expert Opinion

Carneades Screen Shot

ArguMed: Two Types of Corroborative Evidence

Schum (1994): Two Forms of Corroborative Evidence

Supportive Corroborative Evidence in BAC Case

Convergent Corroborative Evidence in BAC Case

Expert Testimony with Double Counting

Corroborative Expert Opinion Evidence in Carneades

A Third Category?

In addition to linked and convergent arguments, there is a third category called EvAcc arguments.

An evidence accumulating argument is one where the probative weight of the conclusion increases as you go from each premise to the next.

Example: in diagnosis of diseases, each premise indicating the presence of a symptom give only a small probative weight to the conclusion that the patient has the disease, but as you go from each premise to the next the probative weight of the conclusion increases [step by step].

An Example from MYCIN

Prem1: The gram stain of the organism is gramneg. Prem2: The morphology of the organism is rod. Prem3: The aerobicity of the organism is anaerobic. Conclusion: There is suggestive evidence that the

identity of the organism is bacteroides. Each premise gives a small probative weight for the

conclusion, but as you go from each premise to the next, that probative weight increases until finally the argument is a “clincher” that gives enough evidence for a diagnosis.

Classifying EvAcc Arguments

They seem more like convergent arguments, because each reason is independent from the next as evidence.

But they also seem like linked arguments, because the premises go together in a cumulative buildup of evidence.

This may be a third category, but how should it be visually represented?

Hypothesis on How to Represent EvAcc Arguments

Maybe EvAcc arguments can be modeled using standards of proof.

According to this hypothesis, each premise is a sign, and as each argument from sign is put forward greater probative weight is gained.

Finally, the probative weight in support of the conclusion meets a standard of proof.

Some Useful Resources

Argument Mapping Software Araucaria: http://araucaria.computing.dundee.ac.uk/ ArguMed: http://ai.rug.nl~verheij/ Rationale: http://rationale.austhink.com/ Carneades: http://carneades.berlios.de/downloads/

Origins of Evidence Diagramming John H. Wigmore, The Principles of Judicial Proof, Boston,

Little, Brown and Company, 1913.

Argumentation Schemes Douglas Walton, Chris Reed and Fabrizio Macagno,

Argumentation Schemes, Cambridge, Cambridge University Press, 2008.