Specificity of Association and Apportionment
Most medical conditions are multifactor in etiology; ie, they are reflective of more than one physiologic or environmental process. Evaluation of the results of epidemiologic studies via regression analysis (which weighs the contribution of various factors both individually and the combination on a given event or events) provides data that can be applied when reaching conclusions regarding the degree to which one would expect a given factor to contribute to the medical condition under evaluation but never can provide definitive answers regarding apportionment.
It is impossible to accurately evaluate to what extent a given factor or exposure was the contributing cause in a multifactorial disease process. Likewise, in certain situations (such as heart disease), the genetic predisposition of the affected individual is a considerable, if not primary, determinant of causality. Assessing the specificity of association is consequently often the most difficult aspect of causality analysis.
There is a great deal of legal terminology focused on establishing, and labeling, the degree to which an event or injury has led to a particular outcome. The legal determination of causality uses, but does not necessarily rely upon, the medical evidence that supports or refutes a casual relationship between a given event and outcome. Furthermore, it is often societal decision, and not scientifically required to reach conclusions about the presence or absence of a casual relationship. Using the legal definitions of causality, a relationship between an event and a given outcome is classified as “probably” or “possible.” It is probable if the chance of them being related is greater than 50%. It is possible if the chance of a relationship is deemed to be less than 50%. The skill of the attorneys arguing the case, the credibility of the claimant and his or her physician, the ability of the medical expert to present the medical information regarding causality, and the existence of case law (which may have established the de facto existence of a casual relationship unless definitively proven otherwise) all influence the ultimate determination. Statements are often made regarding the probability or possibility of a causal relationship between an event and an outcome in the absence of an objective epidemiologic or biologic rationale for the determination. While the Supreme Court, in Daubert case, held that testimony must be grounded in the methods and procedures of science and based on more than simply subjective belief or unsupported speculation to be held as relevant and reliable, this standard is not routinely used in many jurisdictions. Thus the use of the legal terminology alone can imply a degree of certainty that may be completely unfounded.