The natural history group is often referred to as the third arm of a controlled drug trial, from the simple notion that a trial has three arms.
The observed outcomes within this group are then compared with the outcomes manifested by a group that has been given the active drug, and with that manifested by a second group who have been given a dummy, placebo drug (thus, the natural history group is the trial's "third arm").
Read more about this topic: Natural History Group
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