Has a medical doctor ever said to you "You are comparing apples to oranges?" It's a common phrase so what do they mean?
It means that what you are trying to compare maybe similar but not so similar it is fair to compare them. In medical outreach, this would usually come up during a conversation where you try to compare one research paper to another to make a claim.
For example: You have one research paper that shows chiropractic is effective for managing headaches in 80% of patients treated. In another research paper it shows that physical therapy is effective for managing headaches in 50% of patients treated.
If you try to make a statement that chiropractic is more effective than physical therapy for managing headaches the doctor would rightfully state that you are comparing apples to oranges.
For comparisons to be made correctly, they must stem from the same research trial or be a meta-analysis otherwise you're just comparing apples to oranges.