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Attributable Fraction : Fundamental Differences in Interpretations of Probability of Causation

Understand the differences between scientific and legal definitions of causation and the reasons for these differences. Learn why the term "attributable fraction" is often incorrectly equated with the probability of causation.

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Attributable Fraction : Fundamental Differences in Interpretations of Probability of Causation

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  1. Attributable Fraction:Fundamental Differences in Interpretations of Probabilityof Causation Adrienne S. Ettinger, M.P.H. Randi A. Paynter, M.S.

  2. Learning Objectives • To understand the fundamental differences in the scientific and legal definitions of causation • To examine the underlying reasons for these differences • To recognize that the term attributable fraction is often incorrectly equated with the probability of causation

  3. Performance Objectives • To recognize how sound interpretation of epidemiologic principles may be used to enlighten legal determinations regarding individual risk • To recognize the limitations of epidemiologic data in assigning blame

  4. Problem 1) Establishing a causal relationship between exposure and disease can be very complex 2) There are substantial differences in the reasoning processes and proof required in defining causation in science and the law

  5. Underlying Reasons for Differences in Interpretation • Purpose • Requirements for proof • Philosophy

  6. Purpose To examine causes for distribution and determinants of disease Populations or categories of occurrence Purpose To resolve disputes in a fair and impartial manner based on the evidence at hand Individuals or individual events Science Law

  7. Purpose To examine causes for distribution and determinants of disease Populations or categories of occurrence Purpose To resolve disputes in a fair and impartial manner based on the evidence at hand Individuals or individual events Science Law

  8. Requirements for Proof Biologic mechanism Person, place, time Data-driven Requirements for Proof Evidence includes expert testimony Rational basis for expert’s opinion Science Law

  9. Requirements for Proof Biologic mechanism Person, place, time Data-driven Requirements for Proof Evidence includes expert testimony Rational basis for expert’s opinion Science Law

  10. Philosophy Inductive reasoning Hypotheses Universal Degrees of causation Philosophy Deductive reasoning Precedents Jurisdictional No degrees of causation allowable Science Law

  11. Philosophy Inductive reasoning Hypotheses Universal Degrees of causation Philosophy Deductive reasoning Precedents Jurisdictional No degrees of causation allowable Science Law

  12. Attributable Fraction ? Probability of Causation =

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

  14. “More-likely-than-not” Rule • The legal precedent has been that if the probability of causation exceeds 50%, the exposure is “more likely than not” to have caused disease in the individual • In theory, the epidemiologic concept of etiologic fraction corresponds to the legal concept of probability of causation

  15. Attributable fraction:A family of concepts 1) excess fraction 2) etiologic fraction 3) incidence-density (rate) fraction

  16. Does a case occur by time ‘t’ ? excess fraction • policy and planning • effectiveness of treatment

  17. Etiologic fraction~ probability of causation Not usually estimable • Except under certain circumstances when rate fraction approximates EF

  18. not ‘excess’ cases non- exposed cases A0 A1 A2 exposed cases A0 A1 A2 t0 t1 excess cases etiologic cases = A1 + A2

  19. Incidence Rate Fraction Incidence rate if exposed Incidence rate if not exposed Incidence rate if exposed

  20. Doubling Dose • Dose at which the incidence density ratio is 2 or incidence density fraction is 0.5 • Should not be equated with the dose at which the probability of causation is 0.5

  21. Are there alternative measures to attributable fraction?

  22. When does a case occur within time period ‘t’ ? Direct measures of exposure effects on incidence time • Example: years life lost(YLL)

  23. Conclusions • Probability of causation is theoretically equivalent to the concept of etiologic fraction • Etiologic fraction cannot be estimated from observed epidemiologic data in the absence of a biologic mechanism • Attributable fraction refers to a family of concepts, not a single quantity

  24. Further Reading Greenland S. Relation of Probability of Causation to Relative Risk and Doubling Dose: A Methodological Error That Has Become a Social Problem. Am J Public Health 1999;89:1166-9. Greenland S and Robins JM. Epidemiology, Justice, and the Probability of Causation. Epidemiology (In press). Greenland S and Robins JM. Conceptual Problems in the Definition and Interpretation of Attributable Fractions. Am J Epidemiol 1988;128:1186-97. Henderson TW. Toxic Tort Litigation: Medical and Scientific Principles in Causation. Am J Epidemiol 1990;132:S69-S78.

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