Abstract: Developing methods of automated inference that are able to provide users with compelling human-readable justifications for why the answer to a question is correct is critical for domains such as science and medicine, where user trust and detecting costly errors are limiting factors to adoption. One of the central barriers to training question answering models on explainable inference tasks is the lack of gold explanations to serve as training data. In this paper we present a corpus of explanations for standardized science exams, a recent challenge task for question answering. We manually construct a corpus of detailed explanations for nearly all publicly available standardized elementary science question (approximately 1,680 3rd through 5th grade questions) and represent these as"explanation graphs"-- sets of lexically overlapping sentences that describe how to arrive at the correct answer to a question through a combination of domain and world knowledge. We also provide an explanation-centered tablestore, a collection of semi-structured tables that contain the knowledge to construct these elementary science explanations. Together, these two knowledge resources map out a substantial portion of the knowledge required for answering and explaining elementary science exams, and provide both structured and free-text training data for the explainable inference task.

Introduction: Question answering (QA) is a high-level natural language processing task that requires automatically providing answers to natural language questions. The approaches used to construct QA solvers vary depending on the questions and domain, from inference methods that attempt to construct answers from semantic, syntactic, or logical decompositions, to retrieval methods that work to identify passages of text likely to contain the answer in large corpora using statistical methods. Because of the difficulty of this task, overall QA task performance tends to be low, with generally between 20% and 80% of natural (non-artificially generated) questions answered correctly, depending on the questions, the domain, and the knowledge and inference requirements. Standardized science exams have recently been proposed as a challenge task for question answering BIBREF0 , as these questions have very challenging knowledge and inference requirements BIBREF1 , BIBREF2 , but are expressed in simple-enough language that the linguistic challenges are likely surmountable in the near-term. They also provide a standardized comparison of modern inference techniques against human performance, with individual QA solvers generally answering between 40% to 50% of multiple choice science questions correctly BIBREF3 , BIBREF4 , BIBREF5 , BIBREF6 , BIBREF7 , and top-performing ensemble models nearly reaching a passing grade of 60% on middle school (8th grade) science exams during a recent worldwide competition of 780 teams sponsored by the Allen Institute for AI BIBREF8 . One of the central shortcomings of question answering models is that while solvers are steadily increasing the proportion of questions they answer correctly, most solvers generally lack the capacity to provide human-readable explanations or justifications for why those answers are correct. This “explainable inference” task is seen as a limitation of current machine learning models in general (e.g. Ribeiro et al., Ribeiro2016), but is critical for domains such as science or medicine where user trust and detecting potentially costly errors are important. More than this, evidence from the cognitive and pedagogy literature suggests that explanations (when tutoring others) and self-explanations (when engaged in self-directed learning) are an important aspect of learning, helping humans better generalize the knowledge they have learned BIBREF9 , BIBREF10 , BIBREF11 . This suggests that explainable methods of inference may not only be desirable for users, but may be a requirement for automated systems to have human-like generalization and inference capabilities. Building QA solvers that generate explanations for their answers is a challenging task, requiring a number of inference capacities. Central among these is the idea of information aggregation, or the idea that explanations for a given question are rarely found in a contiguous passage of text, and as such inference methods must generally assemble many separate pieces of knowledge from different sources in order to arrive at a correct answer. Previous estimates BIBREF2 suggest elementary science questions require an average of 4 pieces of knowledge to answer and explain those answers (here our analysis suggests this is closer to 6), but inference methods tend to have difficulty aggregating more than 2 pieces of knowledge from free-text together due to the semantic or contextual “drift” associated with this aggregation BIBREF12 . Because of the difficulty in assembling training data for the information aggregation task, some have approached explanation generation as a distant supervision problem, with explanation quality modelled as a latent variable BIBREF7 , BIBREF13 . While these techniques have had some success in constructing short explanations, semantic drift likely limits the viability of this technique for explanations requiring more than two pieces of information to be aggregated. To address this, here we construct a large corpus of explanation graphs (see Figure 1 ) to serve as training data for explainable inference tasks. The contributions of this work are are:

Question: What does it mean for sentences to be "lexically overlapping"?
Answer: They share words.