Table 1.

Comparison of study design attributes in prospective vs. retrospective studies using electronic health records.

	Prospective Study	Retrospective Study with EHR
Selecting Research Participants	Inclusion and exclusion criteria prospectively applied Volunteers willing to undergo study procedures and be actively followed	Inclusion and exclusion criteria applied post hoc Patrons of a healthcare organization Preferably exceed some minimal information threshold
Baseline	Often date of some salient health event such as diagnosis of a condition, procedure performed, or date informed consent provided among generally healthy volunteers	Any time on or between first and last EHR-documented encounters Preferably on the date of an actual encounter Preferably not first encounter
Assembling Baseline Characteristics	At baseline, measured by questionnaires, blood tests, imaging, and other measurement instruments in a standardized manner among all study participants	Only considers data elements collected during usual care Uses encounters occurring on or before the specified baseline Qualitative characteristics determined by rules for the 99%
Follow-up for Future Outcomes	Active, standardized follow-up at regular intervals in all study participants Adjudication of claimed outcomes	Passive follow-up Only study outcomes documented at study institution are identified Rules for the 99% apply