probability help !!!!!

FrostedNipple · 5 Jun 2007 at 10:56

I've got approx 1.5 hours to learn all the rues and stuff as i have an as stats test at 1
P(A)
P(B)
P(A n B)
P(A u B)
P(A|B)
etc etc what are the rules lots of help simplified into bullet points would be very helpful

thanks in advance

Majago · 5 Jun 2007 at 11:08

P(A) = probabilty of A occuring
P(B) = probabilty of B occuring
P(A n B) = probabilty of A and B occuring = P(A)*P(B) if A&B are independant
P(A u B) = probabilty of A or occuring = P(A) + P(B) - P(A and B)
P(A|B) = probability of A given B = probability that A occurs given that B has already occured.

If independant then P(A|B) = P(A) - and vice versa - because the occurrence of one doesn't change the probability of the occurrence of the other.

Also:

P(A and B) = P(A) * P(B|A)

Hope this is a) correct and b) some help

Zefan · 5 Jun 2007 at 11:10

Sounds right ^

Tommy B · 5 Jun 2007 at 11:36

If the events are mutually exclusive/independent, it means that they do not affect each other. Thus,

P(A and B) is simply P(A) X P(B)
P(A or B) is simply P(A) + P(B)

If they DO affect each other then the events are dependent.

P(A or B) is P(A) + P(B) - P(A and B)

P(A and B) is P(A) * P(B|A) ' which means P(A) multiplied by the probability of B , given A.

NB: P(B|A) = P(A and B) / P(A)

FrostedNipple · 5 Jun 2007 at 11:59

thanks guys, should hopefully get those questions right now

Tommy B · 5 Jun 2007 at 16:13

If you were doing OCR S1 you needn't have worried about all the dependence/mutually exclusive crap as it wasn't needed.

Ricochet J · 5 Jun 2007 at 16:44

I had a the Stats test today. Just come back from it. It was funny! I don't know why, I was grinning at it!

One of the questions was something along the lines of

"Musicians want to travel in an aeroplane. One musician has a normal distribution with mean (some random number) and standard deviation (another random numer). Work out:

a) How heavy the musician's bag is

b) What instrument he plays"

LoL! Just LoL! HTF am I meant to know what instrument the guy plays? I got the answer to part a something like 1200 kg, so I wrote Grand Piano!