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 the rule tree below to model the compensation decisions included in our Vaccine/Injury Project (“V/IP”).
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 partial rule tree for the statutory rules governing compensation claims for vaccine-related injuries under the National Vaccine Injury Compensation Program (“VICP”) in the United States. (This image gives an impression of the entire tree, but is not readable; for a scalable, readable version of the entire tree, click on the image.) The VICP, which became effective on 1 October 1988, was created by the National Childhood Vaccine Injury Act of 1986, Pub.L. No. 99-660, Title III, 100 Stat. 3755 (1986) (codified at 42 U.S.C. §§ 300aa-10 – 300aa-34 (2006)).
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 (for example, entering a judgment that “the petitioner is entitled to compensation under the National Vaccine Injury Compensation Program”). 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 – in the Vaccine Act cases, usually a Special Master) 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.
The LLT Research Lab uses software developed by Apprentice Systems, Inc. to create a single rule tree capturing those rules that are the same for all of the decisions that we will model in our study sample, with the tree focused on those sub-rules that are the target of our investigation. For example, in our Vaccine/Injury Project, our rule tree diagram (shown above) captures the statutory rules that are the same for all the decisions in our sample, and it is focused on the rules for proving causation in fact in so-called “Off-Table” cases.
You can take a quick video tour of the Vaccine Act rule tree.
For more details about the logical connectives found within our rule trees, see our post entitled “Rule Tree Diagrams.”