The statistical number-crunching showed that those who drank two to three cups of coffee a day were the least likely to develop mental health problems, compared to people who didn't drink coffee at all or who drank more than three cups.

How much is a cup? Is it the cooking unit equivalent to 250ml or is it a colloquial "cup" that could be anything from the size of a thimble to the size of your head?

If you're going to report on a study about "how much is optimal" can you at least be clear on the units?