Actually that is either a prospective trial that is observational, or a retrospective observational study — both have some validity. But the ONLY way to quantify it is to perform an RCT
There are some things an RCT is unethical for — ie RCT on needing a parachute to jump out of an airplane — if randomized to the placebo group there would be 100% death. As for Ivermectin, there is excellent observational data for it, but nothing conclusive. Its a jump to state that it is unethical not to study the drug prospectively.
Tell that to the Doctor who knows only half his patients are going to receive the single 12 mg dose of scabies medicine that has in his observation helped 100% of covid patients.
http://covexit.com/professor-thomas-borody-interview-part-1/
Australian doctors can prescribe
http://covexit.com/australian-gps-can-legally-prescribe-ivermectin-triple-therapy-protocol-professor-borody/
Inadvertent double blind trial in Toronto nursing home, 100% prevention among 187 treated patients.
https://youtu.be/8XCYzpHBEkI