  Electronic Health Records (EHRs) are typically stored as time-stamped
encounter records. Observing temporal relationship between medical records is
an integral part of interpreting the information. Hence, statistical analysis
of EHRs requires that clinically informed time-interdependent analysis
variables (TIAV) be created. Often, formulation and creation of these variables
are iterative and requiring custom codes. We describe a technique of using
sequences of time-referenced entities as the building blocks for TIAVs. These
sequences represent different aspects of patient's medical history in a
contiguous fashion. To illustrate the principles and applications of the
method, we provide examples using Veterans Health Administration's research
databases. In the first example, sequences representing medication exposure
were used to assess patient selection criteria for a treatment comparative
effectiveness study. In the second example, sequences of Charlson Comorbidity
conditions and clinical settings of inpatient or outpatient were used to create
variables with which data anomalies and trends were revealed. The third example
demonstrated the creation of an analysis variable derived from the temporal
dependency of medication exposure and comorbidity. Complex time-interdependent
analysis variables can be created from the sequences with simple, reusable
codes, hence enable unscripted or automation of TIAV creation.
