Normally the dimensions surrounding a fact table take on a single value in the context of the fact event. However, there are situations where multivaluedness is natural and unavoidable. The diagnosis dimension in healthcare fact tables is a good example. At the moment of a procedure or lab test, the patient has one or more diagnoses. Electronic medical record applications facilitate the physician’s selection of multiple diagnoses well beyond the historical practice of providing the minimal coding needed for reimbursement; the result is a richer, more complete picture of the severity of the patient’s medical condition. There is strong analytic incentive to retain the multivalued diagnoses, along with the other financial performance data, especially as organizations do more comparative utilization and cost benchmarking.If there were always a maximum of three diagnoses, for instance, you might be tempted to create three diagnosis foreign keys in the fact table with corresponding dimensions, almost as if they were roles. However, diagnoses don’t behave like independent roles. And unfortunately, there are often more than three diagnoses, especially for hospitalized elderly patients who may present 20 simultaneous diagnoses! Diagnoses don’t fit into well-defined roles other than potentially the primary admitting and discharging diagnoses.
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