To help determine if your study qualifies as Exempt or Expedited, use this decision tool from the COMIRB website.
Retrospective reviews could qualify as Exempt or Expedited IF they pose minimal risk to participants. If your data is completely de-identified, in other words you are not retaining any of the 18 Patient Health Identifiers, and any link to re-identify is destroyed, your study may qualify as Exempt. If you de-identify your data but retain a re-identifying key, your study most likely qualifies as Expedited.
If your study poses more than minimal risk, you must submit for a Full Board Review.