The original add to make this claim was Trident Sugarless Gum, which basically shopped for dentists to make the claim and stopped when they had four who agreed, and one who didn't. And in fact even then, it was still playing with the exact meaning.
The original quote is "Four out of Five Dentists recommend Trident Sugarless Gum to their patients who chew gum." They left out some critical information. First, that the question the doctors were asked was "would you recommend this product if the patient insists on chewing gum?" That insistent patient changed their answer fundamentally... as they would have joined Doctor Number 5, who recommended never chewing gum, even if Trident Sugarless Gum was the least unhealthy gum too chew... cause if the patient is going to to take a full recommendation on gum chewing, any dentist worth their salt would recommend you not chew gum.
This appeared in the mid to late 1950s and America instantly realized that 4 dentists out of five who answered a poorly phrased question badly does not speak for the bulk of Dentists who saw the answer for what it is. By 1960, at least one Variety Show parodied the commercial by making a gag commercial for a Chinese Restaurant that insisted that "9 out of 10 doctors recommend eating Chinese food." Cue the Camera focusing on 9 happy looking Chinese doctors and one scowling White Doctor as their panel.
As a final note and because I'd be re-missed not putting this in here, 9 out of 10 Doctors insist the 4th Doctor's scarf was way too long.