Guidelines for Annotating Temporal Information

Guidelines for Annotating Temporal Information Inderjeet Mani, George Wilson The MITRE Corporation, W640 11493 Sunset Hills Road Reston, Virginia 20190-5214, USA +1-703-883-6149 imani@mitre.org Lisa Ferro The MITRE Corporation, K329 202 Burlington Road, Rte. 62 Bedford, MA 01730-1420, USA +1-781-271-5875 lferro@mitre.org Beth Sundheim SPAWAR Systems Center, D44208 53140 Gatchell Road, Room 424B Sand Diego, CA 92152-7420, USA +1-619-553-4195 sundheim@spawar.navy.mil ABSTRACT This paper introduces a set of guidelines for annotating time expressions with a canonicalized representation of the times they refer to. Applications that can benefit from such an annotated corpus include information extraction (e.g., normalizing temporal references for database entry), question answering (answering “when” questions), summarization (temporally ordering information), machine translation (translating and normalizing temporal references), and information visualization (viewing event chronologies). Keywords Annotation, temporal information, semantics, ISO-8601. 1. INTRODUCTION The processing of temporal information poses numerous challenges for NLP. Progress on these challenges may be accelerated through the use of corpus-based methods. This paper introduces a set of guidelines for annotating time expressions with a canonicalized representation of the times they refer to. Applications that can benefit from such an annotated corpus include information extraction (e.g., normalizing temporal references for database entry), question answering (answering “when” questions), summarization (temporally ordering information), machine translation (translating and normalizing temporal references), and information visualization (viewing event chronologies). Our annotation scheme, described in detail in [Ferro et al. 2000], has several novel features: • It goes well beyond the one used in the Message Understanding Conference [MUC7 1998], not only in terms of the range of expressions that are flagged, but, also, more importantly, in terms of representing and normalizing the time values that are communicated by the expressions. • In addition to handling fully-specified time expressions [e.g., September 3rd, 1997), it also handles context-dependent expressions. This is significant because of the ubiquity of context-dependent time expressions; a recent corpus study [Mani and Wilson 2000] revealed that more than two-thirds of time expressions in print and broadcast news were context-dependent ones. The context can be local (within the same sentence), e.g., In 1995, the months of June and July were devilishly hot, or global (outside the sentence), e.g., The hostages were beheaded that afternoon. A subclass of these context-dependent expressions are ‘indexical’ expressions, which require knowing when the speaker is speaking to determine the intended time value, e.g., now, today, yesterday, tomorrow, next Tuesday, two weeks ago, etc. Our scheme differs from the recent scheme of [Setzer and Gaizauskas 2000] in terms of our in-depth focus on representations for the values of specific classes of time expressions, and in the application of our scheme to a variety of different genres, including print news, broadcast news, and meeting scheduling dialogs. The annotation scheme has been designed to meet the following criteria: Simplicity with precision: We have tried to keep the scheme simple enough to be executed confidently by humans, and yet precise enough for use in various natural language processing tasks. Naturalness: We assume that the annotation scheme should reflect those distinctions that a human …

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