Citation verification and updating tools are central to effective case law research. Once a case is decided and the opinion published, it does not remain static. It becomes part of a developing body of case law on a particular issue. The individual case may be frequently cited and relied upon by judges considering subsequent cases or it may be disagreed with and distinguished until its precedential value is negligible. Although a case continues to appear in a print reporter or in an electronic database over the years (that is, cases which are overruled or otherwise treated unfavorably are not deleted from publications or databases), its value as precedent is determined by subsequently decided cases which must be discovered through research. This is because judges decide cases based on reasoning of previously decided cases; therefore any individual case can be seen as part of a chain of decisions which develops a legal theory. It is often necessary for thorough research and a reliable conclusion to find and read all or most of the cases that are part of the chain. Depending upon where in the research process you enter this chain - by locating a reference to a relevant case through a secondary source, digest or electronic research, or otherwise - it is crucial to determine where that case stands in relation to the body of case law on that or related topics. Such a comprehensive review of the relevant mandatory authorities usually requires the use of several research sources and techniques. Among the resources which must be consulted are citators.
In the current world of legal research, citators are used primarily in electronic format. There are two main purposes achieved by consulting citators as part of the case research process:
For example, the case may have been cited with approval or followed by many later cases, indicating that its holding carries strong precedential weight. Alternatively, the case may have been frequently criticized or limited by later cases, which weakens its value as precedent. Less frequently, a case may be expressly overruled in a later opinion, indicating that the court will no longer follow it as precedent.