system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do they determine statistics like “8 million people in the US have __ disease and another 1 million are undiagnosed”?

I work in healthcare and there have been multiple times where I’ve seen disease prevalance statistics that include “undiagnosed cases”. If they have not been diagnosed then where do they get those numbers from?
gpt
By reliably measuring rate of infection in smaller populations then extrapolating the results to larger populations, researchers compare the extrapolation against reported cases. The difference between these quantities becomes the assessment of undiagnosed cases, which is always subject to some measure of error.