The overruling of prior case law is one of the most dramatic events in a common-law system, and the rate of overrulings is often considered an important measure of legal change. To measure this process more precisely, and thereby elucidate the relationship between legal doctrines of stare decisis and judicial behavior, software was written for the rapid collection of all binding precedents within a jurisdiction in a database, with their time of origin, indicator variables of interest, and their time and method of reversal or overruling. The present prototype study involved the extraction of the fifty thousand substantive opinions of the Illinois Supreme Court between 1819 and 2005. From this study population, 733 cases have "failed," becoming no longer good law, of these, 504 cases have thus far been overturned by the court itself, a long-term trend that suggests the overall chance of being overruled converges toward approximately 1%. Certain types of precedents, such as those issued with a dissent, or having a constitutional aspect, are at significantly greater risk, and the more recent periods of court history expose all precedents passing through them to elevated hazard. Application of engineering and epidemiological techniques of hazard analysis allows specification of the time-dependent risk of failure experienced by a precedent. In addition, any concentration of precedent "mortality," when controlled for intrinsic characteristics of the precedents at risk, is an indicator of an unusually hazardous judicial environment. In Illinois, a risky environment appears in the middle of the twentieth century, and is correlated with a change in the state’s doctrine of stare decisis, which began to de-emphasize the protective value of a precedent’s age. A simple model relates the apparent effect of the exposing older precedents to greater risk to a spike in legal instability, followed by a higher equilibrium rate of court overruling. Hazards regression and related statistical techniques, combined with automated generation and coding of citation databases, appears to be a promising method for diagnosis of legal dynamics, understanding episodes of legal change, and estimating the relative likelihood that a current rule of law will eventually be overturned
