Apparently more beneficiaries have to undergo a Long Form CDR based on being randomly selected, than because their Short Form triggered a Long Form CDR.

https://secure.ssa.gov/poms.nsf/lnx/0440502001

"When a mailer is completed, there are two possible outcomes depending on the information received. One outcome is a “DO CDR” (mailer to be referred for a full medical CDR) that results in a full medical CDR that occurs in about 2.5% of the completed mailers. The other outcome is to defer doing a full medical CDR, resulting in a mailer deferral action."

"As a result of integrity sampling, approximately 5% of the mailers received are referred for processing as full medical CDRs despite responses which may otherwise indicate that a deferral action is proper."


https://secure.ssa.gov/poms.nsf/lnx/0428015310

Age and length of time collecting SSI/SSDI are factored when determining a beneficiary's RFC during the CDR process.

Also, the Short Form CDRs (which really aren't CDRs but that's how they're generally referred to) are sent from Wilkes-Barre, where as Long Form CDRs (full medical reviews) are sent from your state DDS (or sometimes another state's DDS).