In the metro station, there is this screen where you can see all kinds of ads and some this-and-that kind of news. One of the regular features is something called “number of the day” from http://www.zahlenwelt.net (note that I live in Germany). Today’s “number” was:
“For moviegoers, the probability of watching a movie alone is 7%”
That seemed somehow strange, given that I am a regular moviegoer, having seen probably around 50 films in the last two years, but never once alone. Ok, you might say that I am a victim of the “Availability heuristic“, having not taken into account that other people might be different. But of course I know that there are people going to the movies alone.
I think that the quote sounds somehow wrong because I am so used to thinking in terms of conditional probabilities, and so I interpreted the quote as: “Given someone is a moviegoer (M), his/her probability of watching a movie alone (A) is 7%” or P(A|M)=7%. Now even if that is right, it is not very informative stated in that way, because there will be many M like me who rarely if ever watch a movie alone, and some M who do so regularly. What has happened is probably that they have just counted in a sample of cinemas and movies how many people were watching alone, and they added the qualifier “Among moviegoers” to show that they are not talking about non-moviegoers at all.