"As I mused, the fire burned"

Reflection on life as a person of faith.

Legal versus medical causation

leave a comment »

Causation: Legal vs. Scientific Perspectives or How to tell if A caused B…

A medical specialist says that it is “possible, but not probable” that an accident caused a particular injury: does this meet the threshold for the legal but-for test or the material contribution test, greater than de minimus? Workers’ Compensation cases often turn on the legal interpretation of medical evidence, so it is interesting to consider if medicine and the law speak the same language.

Physicians are trained to use the scientific method when offering opinion about causal relationships. Science asks for a very high level of probability before concluding a causal relationship might exist. Science usually assesses relationship based on a two sigma level of confidence (we can discuss this over coffee if you’re a fan of statistics). For a normally distributed population, this means that you can say with confidence that 95% of data points will fall inside a given range. When calculating ballistics, the military drops test bombs until there are enough data points to say, with 95% confidence, where a given bomb would hit. A bomb landing outside the “normal” range, one of the 5%, would be considered, “possible, but not probable.”

The two-sigma level is a very high standard, which likely even far surpasses the criminal test of beyond a reasonable doubt (some have set that limit at about 0.70 to 0.74, although some would say there can be no number assigned to the legal question… “Proof beyond a reasonable doubt is a quantum without a number.”) However, there is risk in making such comparisons, for the legal and the scientific tests for causation are based on entirely different sets of assumptions. A straight numerical comparison, 51% versus 95%, for example, will miss those critical differences.

Consider a person who has a blood test with an abnormal result that might be the result of a work accident. If this was a hearing, would you consider that abnormal result to be proof of an injury: but for the accident or beyond a de minimus level? “Normal” for blood tests is established to a two-sigma level by science: 95% of the population’s blood test result will fall within the “normal” range.

However, “normal” excludes about 5% of the population who have blood chemistry normally outside that of 95% of the population (who, we might say, are normally abnormal). If you ask a physician about that blood test, she would say that a healthy value outside the “normal” range is “possible, but not probable.”

A decision-maker needs to be aware of the difference between the two approaches to causation when weighing medical opinion. The medical professional offering the opinion is not thinking (or at least, should not be thinking) about writing to satisfy the legal test, but rather the scientific standard. “Possible, but not probable” may suffice to meet a legal test, depending on the fact picture in play, the legal test being applied, and the totality of the evidence.

A recent case, Libby Wallace 2010 ABQB 368, at paragraph 86-87, stated this distinction with respect to medical panel findings, “…while it is the medical panel’s function to decide the medical issues, it is the appeal body’s function to apply the law to determine whether an injury was caused by a compensable work accident…Questions about causation raise some medical issues that are within the purview of a medical panel, but there is a distinction between scientific and legal findings. This distinction exists both in common law (see Laferrier v. Lawson, [1991] 1 S.C.R. 541 at para. 156) and in workers compensation case law. At law, causation need not be determined with precision, but must meet the appropriate burden of proof…On the other hand, scientific analysis of causation seeks a far higher degree of certainty to establish causation.”

When a physician states something is “possible, but not probable” they do so with that scientific standard in mind, and not the legal test. “Probable”, for a scientist, indicates certainty at a 95% level of confidence; “possible” means something less.

To summarize the differences between the legal and scientific approaches to causation:
– science deals with population-based analysis (general)
– the law is concerned with the individual case (particular)
– science deals with probabilities (the uncertainty of the truth)
– the law is concerned with a single event or group of events (the absolute certainty of the truth)
– science does not seek finality, but is an ongoing quest for new understanding; all theories are tentative
– the law seeks finality (there is no res judicata or collateral estoppel in science, climate change advocates apart)
– science’s goal is to create abstracted models of reality based on empirical evidence (what is)
– law’s goal is more tentative, seeking truth sometimes based on incomplete or inferred evidence, assessed collectively (what ought to be)


Written by sameo416

September 15, 2010 at 3:25 pm

Posted in Uncategorized

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

Urbane Adventurer: Amiskwacî

thoughts of an urban Métis scholar (and sometimes a Mouthy Michif, PhD)

Joshua 1:9

Reflection on life as a person of faith.

Engineering Ethics Blog

Reflection on life as a person of faith.


Today, the Future and the Past all kinda rolled up in one.


For Those Courageous in Standing for Truth


Law, language, life: A Plains Cree speaking Métis woman in Montreal

Malcolm Guite

Blog for poet and singer-songwriter Malcolm Guite

"As I mused, the fire burned"

Reflection on life as a person of faith.

%d bloggers like this: