system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: They say that 650 million people watched the moon landing. How do they take in to account how many people were watching on the same television?
gpt
They don't actually know how many people watched it. With broadcast TV you can't know who is actually receiving the broadcast and whose TV is turned on at that channel.

What they did was simply survey. Ask a large group of people whether they watched the moon landing on TV and extrapolate from there to the entire population. That's basically how TV ratings were measured.