Errors, like straws, upon the surface flow;
He who would search for pearls must dive below.
- John Dryden
Once, when some painting work was going on in our house, a painter stood in my room watching the physiotherapist give me exercises. He asked many questions about my stroke and said that he had heard that if you got a cracking sound when the fingers or toes are pulled, it meant that the limb was normal. The physiotherapist said that it was just the sound of air bubbles popping in a fluid in the joints and was not of great significance. But the painter refused to accept the explanation and started pulling my toes.
I wondered how a person who probably knew nothing about the functioning of the human body could argue so confidently with someone who had studied about it for years. The physiotherapist said that this happened frequently in the hospital. When he would be giving exercise to a person, somebody will come up and say that some other exercise should be given.
Over the years I decided on a rule of thumb for making out whether a person knew what he was talking about. He will use words like 'it depends', talk about side effects, likely complications, failure rates etc. As David Quammen said in The Boilerplate Rhino:
Having had many chances to study scientists as they study nature, I've seen that science itself is a fallible human activity, not a conceptual machine-tool, and that while accuracy and precision can be easily achieved, validity and meaning cannot. The imperfections and constraints vitiating scientific knowledge stand as a warning about the limits of other sorts of knowledge - even shakier sorts - including that based on eyewitness experience. Moral: We live in a tricky universe, and it behooves us to be just a bit provisional about our convictions.
Carl Sagan wrote in The Demon-Haunted World:
Humans may crave absolute certainty; they may aspire to it; they may pretend, as partisans of certain religions do, to have attained it. But the history of science - by far the most successful claim to knowledge accessible to humans - teaches that the most we can hope for is successive improvement in our understanding, learning from our mistakes, an asymptotic approach to the Universe, but with the proviso that absolute certainty will always elude us.We will always be mired in error. The most each generation can hope for is to reduce the error bars a little, and to add to the body of data to which error bars apply. The error bar is a pervasive, visible self-assessment of the reliability of our knowledge.
On the other hand a person with a tenuous grasp of the subject being discussed, being unencumbered by any knowledge of the subtleties involved, will try to sell you lemon juice giving you 'hundred percent guarantee'. As H. L. Mencken said, "For every complex problem there is an answer that is clear, simple, and wrong."
Unfortunately it seems that we’re swayed by confidence more than expertise.(I know, I know you are not one of those. It is about others.) Many people also have a poor grasp of probability. (I am not very good at it. I keep getting surprised by the answers to various questions.) Perhaps Arthur Benjamin's suggestion needs to be considered.