A question answering system that determines whether a question is off-topic by performing the following steps: (i) receiving, by a question answering system, a set of documents; (ii) identifying topical subset(s) for each document of the set of documents using named entity recognition, where each topical subset relates to a corresponding topic; (iii) assigning a set of topic score(s) for each topical subset using natural language processing, where each topic score relates to a corresponding probability associated with the respective topical subset under a probabilistic language model; and (iv) determining, based, at least in part, on the topic score(s) corresponding to the topical subset(s), whether a question input into the question answering system is off-topic.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A method comprising: receiving, by a question answering system, a set of documents; identifying topical subset(s) for each document of the set of documents using named entity recognition, where each topical subset relates to a corresponding topic; assigning a set of topic score(s) for each topical subset using natural language processing, where each topic score relates to a corresponding probability associated with the respective topical subset under a probabilistic language model; and determining, based, at least in part, on the topic score(s) corresponding to the topical subset(s), whether a question input into the question answering system is off-topic. 2. The method of claim 1, wherein each set of topic scores includes an on-topic score and an off-topic score, the on-topic score relating to a probability associated with the respective topical subset under an on-topic probabilistic language model, and the off-topic score relating to a probability associated with the respective topical subset under an off-topic probabilistic language model. 3. The method of claim 1 wherein the determining includes assigning a topic score to the question input into the question answering system. 4. The method of claim 3, wherein the topic score assigned to the question is determined, at least in part, by using a sigmoid function to model the difference between a logarithm probability associated with the question under the on-topic probabilistic language model and a logarithm probability associated with the question under the off-topic probabilistic language model. 5. The method of claim 3, wherein the determining further includes comparing the question's assigned topic score to a relevance score associated with an answer generated by the question answering system in response to the question. 6. The method of claim 5, wherein: the question is determined to be off-topic if the question's assigned topic score is greater than the generated answer's associated relevance score; and the question is determined to be on-topic if the question's assigned topic score is less than the generated answer's associated relevance score. 7. The method of claim 1, further comprising: responsive to determining that the question is off-topic, informing a user who input the question into the question answering system that the question is off-topic. 8. The method of claim 1, wherein the determining includes identifying trigram(s) and associated trigram feature(s) for the question input into the question answering system. 9. The method of claim 8, wherein the trigram feature(s) include at least one of the following: lemma, meaning, type, focus, and/or lexical attribute type.