One of the primary goals in differential diagnosis is to definitively rule out as many hypotheses as possible in order to concentrate on the most probable set of diagnoses. One study (Chapman et al., 2001a) estimated that between 40% and 80% of all findings were explicitly negated in ten different report types, with surgical pathology and operative notes demonstrating the least amount of negation and mammograms and chest radiograph reports demonstrating the most. Explicit negations are indicated by negation terms such as "no,'' "without,'' and "denies.'' Findings can also be implicitly negated. For example, "The lungs are clear upon auscultation'' indicates that rales/crackles, rhonchi, and wheezing are all absent. We focus on explicit negation, which is the most common type of negation in patient reports.

In most cases, a physician can easily determine from a report whether a finding is negated in the text. In the sentence, "The patient denies chest pain but has experienced shortness of breath,'' a physician would assign the clinical variable chest pain the value of no and the variable shortness of breath the value of yes. The types of information a human uses to identify explicitly negated findings include (1) negation terms, (2) scope of the negation term, and (3) expressions of uncertainty.

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