Diagrams of rule trees enable users to visualize the logic of a system of legal rules. The Lab creates software models that both display as rule tree diagrams and provide active templates for modeling the reasoning of individual legal decisions. We use this particular rule tree to model the U.S. medical malpractice decisions included in our Medical Malpractice (“MedMal”) Project.
Rules of law state the conditions (procedural and substantive) under which particular types of governmental action are justified. We can visualize systems of legal rules as inverted “trees” — trees that identify and connect all of the conditions for applying that system of rules. For example, the image below shows a basic rule tree for deciding most medical malpractice cases in the United States. (If the image below is not readable, you may click on the image for a scalable, more readable version of the tree.)
Each node of a rule tree is a proposition, capable of having any one of three truth-values (“true” / “undecided” / “false”). The top node of a tree represents the ultimate issue to be proved before some particular governmental action is justified (in this rule tree, entering a court judgment that “the defendant is liable to the plaintiff for medical malpractice”). Each level of each branch extending downward from the top node states the logical conditions for proving the immediately higher proposition. When a legal proceeding begins, all propositions that form the conditions of the applicable legal rules are “undecided.” Participants in the legal process produce evidence and arguments to persuade the decision-maker (whether judge, regulator, or factfinder) to change the values of those propositions to either “true” or “false.” Put another way, the legal rules identify the propositions that are relevant within the type of proceeding, but the particular proceeding begins with the decision-maker being neutral on whether the conditions for applying those rules are satisfied or not.
One other feature of this image of the rule tree is a display, in the upper left-hand corner, of the four “dynamic global subjects” embedded in the tree. When this tree is used as a template to model the reasoning in a particular case, the modeler would substitute for the words “the plaintiff” the name of the actual plaintiff in that case, and similar substitutions would be made for “the defendant,” “the alleged harm,” and “the alleged conduct.” These substitutions would then occur wherever those phrases occur in the modeled reasoning, and the generic rules are thus applied to the specific case.