Attributable Fraction: Fundamental Differences in Interpretations of Probability of Causation...

Attributable FractionAttributable Fraction:: Fundamental Differences in Fundamental Differences in Interpretations of ProbabilityInterpretations of Probability

of Causationof Causation

Adrienne S. Ettinger, M.P.H.Adrienne S. Ettinger, M.P.H.

Randi A. Paynter, M.S.Randi A. Paynter, M.S.

Learning ObjectivesLearning Objectives

• To understand the fundamental To understand the fundamental differences in the scientific and legal differences in the scientific and legal definitions of causationdefinitions of causation

• To examine the underlying reasons for To examine the underlying reasons for these differencesthese differences

• To recognize that the term To recognize that the term attributable fractionattributable fraction is often is often incorrectly equated with the incorrectly equated with the probability of causationprobability of causation

Performance ObjectivesPerformance Objectives

• To recognize how sound interpretation To recognize how sound interpretation of epidemiologic principles may be of epidemiologic principles may be used to enlighten legal determinations used to enlighten legal determinations regarding individual risk regarding individual risk

• To recognize the limitations of To recognize the limitations of epidemiologic data in assigning blameepidemiologic data in assigning blame

ProblemProblem

1) Establishing a causal 1) Establishing a causal relationship between exposure relationship between exposure and disease can be very complexand disease can be very complex

2) There are substantial differences 2) There are substantial differences in the reasoning processes and in the reasoning processes and proof required in defining proof required in defining causation in science and the lawcausation in science and the law

Underlying Reasons for Underlying Reasons for Differences in Differences in InterpretationInterpretation

• PurposePurpose• Requirements for proof Requirements for proof • PhilosophyPhilosophy

PurposePurpose• To examine causes To examine causes

for distribution and for distribution and determinants of determinants of diseasedisease

• Populations or Populations or categories of categories of occurrenceoccurrence

PurposePurpose• To resolve disputes To resolve disputes

in a fair and in a fair and impartial manner impartial manner based on the based on the evidence at handevidence at hand

• Individuals or Individuals or individual eventsindividual events

ScienceScience Law Law


PurposePurpose• To examine causes To examine causes

for distribution and for distribution and determinants of determinants of diseasedisease

• Populations or Populations or categories of categories of occurrenceoccurrence

PurposePurpose• To resolve disputes To resolve disputes

in a fair and in a fair and impartial manner impartial manner based on the based on the evidence at handevidence at hand

• Individuals or Individuals or individual eventsindividual events

Requirements for Requirements for Proof Proof

• Biologic Biologic mechanism mechanism

• Person, place, Person, place, timetime

• Data-drivenData-driven

Requirements for Requirements for ProofProof

• Evidence includes Evidence includes expert testimony expert testimony

• Rational basis for Rational basis for expert’s opinionexpert’s opinion



PhilosophyPhilosophy• Inductive Inductive

reasoningreasoning• HypothesesHypotheses• Universal Universal • Degrees of Degrees of

causationcausation

PhilosophyPhilosophy• Deductive Deductive

reasoningreasoning• PrecedentsPrecedents• JurisdictionalJurisdictional• No degrees of No degrees of

causation causation allowableallowable

AttributablAttributable e

FractionFraction

Probability Probability of Causationof Causation

==??

Probability of Probability of CausationCausation

In the legal sense, the probability In the legal sense, the probability of causation is the probability of causation is the probability that an exposure contributed to that an exposure contributed to development of disease in an development of disease in an individual or group of individuals, individual or group of individuals, either by initiating or either by initiating or accelerating the disease process.accelerating the disease process.

““More-likely-than-not” More-likely-than-not” RuleRule

• The legal precedent has been that if the The legal precedent has been that if the probability of causation exceeds 50%, probability of causation exceeds 50%, the exposure is “more likely than not” the exposure is “more likely than not” to have caused disease in the individualto have caused disease in the individual

• In theory, the epidemiologic concept of In theory, the epidemiologic concept of etiologic fractionetiologic fraction corresponds to the corresponds to the legal concept of probability of causationlegal concept of probability of causation

Attributable fractionAttributable fraction::A family of conceptsA family of concepts

1) excess fraction1) excess fraction

2) etiologic fraction2) etiologic fraction

3) incidence-density (rate) 3) incidence-density (rate) fractionfraction

Does Does a case occur by time a case occur by time ‘‘t’ t’ ??

excess excess fractionfraction

• policy and planningpolicy and planning

• effectiveness of treatmenteffectiveness of treatment

Etiologic fractionEtiologic fraction~ probability of ~ probability of causationcausation

Not usually Not usually estimableestimable

• Except under certain Except under certain circumstances circumstances when rate when rate fraction approximates EF fraction approximates EF

non-non-exposedexposedcasescases

exposeexposeddcasescases

tt00 tt11

AA00

AA00

AA11

AA11 AA22

AA22

excess excess casescases

notnot ‘excess’ cases ‘excess’ cases

etiologic cases = Aetiologic cases = A11 + + AA22

Incidence Rate FractionIncidence Rate Fraction

Incidence Incidence rate rate

if exposedif exposed

Incidence rateIncidence rateif not exposedif not exposed

Incidence rate if exposedIncidence rate if exposed

Doubling DoseDoubling Dose

• Dose at which the incidence Dose at which the incidence density ratio is 2 or incidence density ratio is 2 or incidence density fraction is 0.5density fraction is 0.5

• Should not be equated with the Should not be equated with the dose at which the probability of dose at which the probability of causation is 0.5causation is 0.5

Are there alternative Are there alternative measures to measures to

attributable fractionattributable fraction ??

WhenWhen does a case occur does a case occur within time period ‘within time period ‘t’ t’ ??

Direct measures of Direct measures of exposure effects on exposure effects on

incidence timeincidence time

• Example: Example: years life lostyears life lost (YLL)(YLL)

ConclusionsConclusions

• Probability of causation is Probability of causation is theoretically equivalent to the theoretically equivalent to the concept of etiologic fractionconcept of etiologic fraction

• Etiologic fraction cannot be estimated Etiologic fraction cannot be estimated from observed epidemiologic data in from observed epidemiologic data in the absence of a biologic mechanismthe absence of a biologic mechanism

• Attributable fractionAttributable fraction refers to a refers to a family of concepts, not a single family of concepts, not a single quantityquantity

Further ReadingFurther ReadingGreenland S. Relation of Probability of Causation to Greenland S. Relation of Probability of Causation to Relative Risk and Doubling Dose: A Methodological Relative Risk and Doubling Dose: A Methodological Error That Has Become a Social Problem. Am J Public Error That Has Become a Social Problem. Am J Public Health 1999;89:1166-9.Health 1999;89:1166-9.

Greenland S and Robins JM. Epidemiology, Justice, Greenland S and Robins JM. Epidemiology, Justice, and the Probability of Causation. Epidemiology (In and the Probability of Causation. Epidemiology (In press).press).

Greenland S and Robins JM. Conceptual Problems in Greenland S and Robins JM. Conceptual Problems in the Definition and Interpretation of Attributable the Definition and Interpretation of Attributable Fractions. Am J Epidemiol 1988;128:1186-97.Fractions. Am J Epidemiol 1988;128:1186-97.

Henderson TW. Toxic Tort Litigation: Medical and Henderson TW. Toxic Tort Litigation: Medical and Scientific Principles in Causation. Am J Epidemiol Scientific Principles in Causation. Am J Epidemiol 1990;132:S69-S78.1990;132:S69-S78.

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