Cultural reference

population $p$, penetration $v$ (consider view count of a YouTube video), friends $f$.

Define: \[ P(\t{seen video}) = v/p
P(\t{your friend}) = f/p \]

You are interested if you can use a cultural reference among your friends, consider this value \[ P(\t{seen video}|\t{your friend}) \]

Assume independence (obviously wrong, however, we have nothing better in the zeroth approach): \[ P(\t{seen video}|\t{your friend}) = v/p \]

Oh, XKCD already got it quantified too.