Objective: To measure the uncertainty of temporal assertions like "3 weeks ago" in clinical texts.
Design: Temporal assertions extracted from narrative clinical reports were compared to facts extracted from a structured clinical database for the same patients.
Measurements: The authors correlated the assertions and the facts to determine the dependence of the uncertainty of the assertions on the semantic and lexical properties of the assertions.
Results: The observed deviation between the stated duration and actual duration averaged about 20% of the stated deviation. Linear regression revealed that assertions about events further in the past tend to be more uncertain, smaller numeric values tend to be more uncertain (1 mo v. 30 d), and round numbers tend to be more uncertain (10 versus 11 yrs).
Conclusions: The authors empirically derived semantics behind statements of duration using "ago," and verified intuitions about how numbers are used.