## 2 Background and Related Work  
### Legal Context: Post-Grant Review Proceedings  
This paper studies a particular kind of litigation: post-grant patent review proceedings in the United States before a specialized tribunal: the Patent Trial & Appeal Board. Briefly, in 2011, the United States Congress passed a law called the Leahy-Smith America Invents Act (AIA), which went into effect on 16 September 2012. The AIA created three new review mechanisms for U.S. patents: inter parties review (IPR), post-grant review (PGR), and covered business methods review (CBM) [20]. These three mechanisms are very similar. In all three, a party files a petition with the U.S. Patent & Trademark Office (PTO) challenging the validity of a U.S. patent based on legal and factual bases, called \"Grounds\", set forth in the petition. The PTO Director then has the discretion by statute to decide whether to institute a trial on the challenge. If instituted, the trial is generally decided within 12 months by a panel of three Administrative Patent Judges (APJs, or for the purposes of this paper we will refer to them as \"judges\") within the Patent Trial & Appeal Board (PTAB) unit of the PTO. In practice, the Director further delegates the authority to initially determine whether to institute a trial to the panel of judges who would later hear and decide the trial if instituted. Figure 1 shows a schematic of the post-grant review proceeding. The patent owner is granted an opportunity to file a preliminary response to the petition, often called a patent owner's preliminary response (POPR). While the preliminary response is optional, it is filed in most cases, and one study showed that, empirically, just filing it increases by 31% the probability of the patent owner ultimately succeeding in the proceeding, in part by increasing the chance of the petition being denied at the initial stage [22]. Within 6 months of the petition being filed, the APJs issue an initial determination. The initial determination  
states whether to grant or deny trial on the petition, and thereby proceed to the next phase of the proceeding. APJs must grant the petition and institute trial if there is a reasonable likelihood of success for any ground of invalidity raised in the petition. The primary differences between IPR, PGR, and CBM proceedings is what patents are eligible, and what invalidity grounds can be asserted. An IPR petition can be filed against any patent by anyone. An IPR, however, must be based solely on grounds that have as their factual basis printed prior art (i.e., patents or publications). Usually IPRs are brought by the challenger that was sued on the patent in a separate federal lawsuit brought by the patent owner. PGR and CBM petitions may, by contrast, be based on additional grounds of invalidity available under U.S. patent law (such as a lack of support for claims in the patent specification). A PGR can be filed against patents within 9 months of it issuing and only on inventions more recent than 16 September 2012. CBM has since been phased out; when it was available, it could only target a \"business method\" patent, and it has been phased out. In this paper, we consider all petitions together. In all cases, usually the challenger filing the petition is an entity that was sued for infringing on the patent in federal court. The rules for post-grant review proceedings have been established by the AIA statute, regulations promulgated by the PTO and precedents set through PTAB decisions, and a number of U.S. Supreme Court decisions since the proceedings began in 2012. Importantly, the rules and judicial tendencies have changed over time--an aspect of the data (a.k.a. concept drift) that has previously been identified as a major challenge for applying machine learning to all kinds of litigation [24]. For example, in April 2018, the U.S. Supreme Court decided in _SAS Institute, Inc. v. lancu_ that if any ground in a petition is deemed to have a reasonable likelihood of success, then trial should be instituted on all grounds.