It is generally implied that “best estimate” is, in part, based on information that the estimator knows of. It is more than a guess, but less than an accurate appraisal.
Below are my created examples for comparison.
I my best estimate is that this house is worth about $200,000.00
(I know the exact values of other similar houses in the area and I am basing it on that definite knowledge. I have checked all the comparable homes and I have come up with this best estimate.)
My best guess this house is worth about $200,000.00.
(I know the general value of the homes in this area. This is a reasonable guess.)
My appraisal of this house finds it worth $200,000.00.
(I am a certified home appraiser. I have hired a home inspector to check on the condition of the home. I have also checked “comps” in the area and using all that information I have come up with this valuation.)
There is no way to count. You can't count 15 million people one by one. So you have to analyze surveys, medical studies, government information from agencies that study that problem, and other public data to figure out the percentage of people affected. These kinds of things are all published in academic journals where the research methods can be analyzed for quality. And from there you can calculate a number. But it will never be exact. It might not even be close. But there's no better research available and no exact count available.