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I dont understand this explanation from my textbook (conditional probabilities)

http://prntscr.com/xp5mqs

Where did the n(%) come from and what does it mean?
Reply 1
Avatar for mqb2766
mqb2766
21
It's used to transform the count to probabilities, so I guess it's the total number of things.
Reply 2
Avatar for davros
davros
16
Original post by TSR360
http://prntscr.com/xp5mqs

Where did the n(%) come from and what does it mean?

I've never seen that notation before (possibly a typo?) but it seems to be used to represent the "total number" so that absolute numbers can be converted into probabilities.
Reply 3
Avatar for TSR360
TSR360
OP
11
Original post by davros
I've never seen that notation before (possibly a typo?) but it seems to be used to represent the "total number" so that absolute numbers can be converted into probabilities.

So if you had n(H and G) & n(G) as percentages then dividing them would be the same as dividing n(H and G) & n(G)?
Reply 4
Avatar for davros
davros
16
Original post by TSR360
So if you had n(H and G) & n(G) as percentages then dividing them would be the same as dividing n(H and G) & n(G)?

I don't think it;s supposed to represent percentages - but I could be wrong. Is the notation used anywhere else in the book?

You're just dividing one absolute number by another one to calculate a probability e.g. if I have a bucket containing 10 balls in total and 3 of them are red then P(picking a red) = n(red) / n(total). I think that's all they've done here.
Reply 5
Avatar for mqb2766
mqb2766
21
Thinking about it a bit more
n (%)
Is usually used to report data for categories/sets where they give the tuple
number in set, percentage of the total
However that can't be meant here so, as davros suggested, probably a typo and the only sensible meaning is total number of things.
Original post by TSR360
http://prntscr.com/xp5mqs

Where did the n(%) come from and what does it mean?


I can see how the % symbol might be used mistakenly in some A-level textbooks, when they mean the universal set.

Untitled3.jpg
I'm also tending towards it originally being either \mathcal{E} or \Omega and at some point in the pipeline something couldn't handle it and replaced it with %.

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