# Thread: General Statistics Help - AS Level Maths

1. ## General Statistics Help - AS Level Maths

Hi,

Lately I've been doing statistics revision and I've realised that I'm not really
fully clear on all of the rules and how they work, hence I was hoping someone might be able to talk me through where I am going wrong.

I think it's maining when it comes to intersect (n) and union (u) which confuses me. I know that intersect is obviously where the 2 events intersect (therefore when something AND something else happen) and union is when one event OR the other occur. I've found no problem in placing these on a venn diagram concerning two events and finding the appropraite values from this.

However when I get questions like "Given that P(A) = 0.7, P(B) = 0.4 and P(AuB) = 0.8 find the following:

a) P(AnB) "

Now for this I would think that it'd be 0.7 x 0.4 as I always remember from lower probability that AND is x and OR is plus. However I find that according the my text book I should be doing:

P(AnB) = P(A) x P(B) - P(AuB)

I recognise this as the addition rule rearranged however I still do not understand exactly why I am using this and why I cannot use the multiplication rule - is it something to do with the fact that it's P(AnB) = P(A|B) x P(B) or something?

I'm still not clear on the workings of all this. Do we change things depending on if an event is independant or is mutually exclusive? If P(AnB) > 0 then is the event independant because it has a cross over between the two?

I just find myself a bit lost and confused at times because I want to follow the old "multiply along the branches" of tree diagrams method and feel I cannot differienciate between when to use one method over another.

Any explanation of where I'm going wrong would be of much help, thank you very much.

2. Originally Posted by hiyarearl
Hi,

Lately I've been doing statistics revision and I've realised that I'm not really
fully clear on all of the rules and how they work, hence I was hoping someone might be able to talk me through where I am going wrong.

I think it's maining when it comes to intersect (n) and union (u) which confuses me. I know that intersect is obviously where the 2 events intersect (therefore when something AND something else happen) and union is when one event OR the other occur. I've found no problem in placing these on a venn diagram concerning two events and finding the appropraite values from this.

However when I get questions like "Given that P(A) = 0.7, P(B) = 0.4 and P(AuB) = 0.8 find the following:

a) P(AnB) "

Now for this I would think that it'd be 0.7 x 0.4 as I always remember from lower probability that AND is x and OR is plus. Mr F says: This is only true if A and B are independent events. In this question they are not.

However I find that according the my text book I should be doing:

P(AnB) = P(A) x P(B) - P(AuB) Mr F says: You have misquoted the formula. The correct formula is P(AnB) = P(A) + P(B) - P(AuB).

I recognise this as the addition rule rearranged however I still do not understand exactly why I am using this and why I cannot use the multiplication rule - is it something to do with the fact that it's P(AnB) = P(A|B) x P(B) or something? Mr F says: The multiplication rule can only be used if it's known that A and B are independent events.

I'm still not clear on the workings of all this. Do we change things depending on if an event is independant or is mutually exclusive? If P(AnB) > 0 then is the event independant because it has a cross over between the two?

I just find myself a bit lost and confused at times because I want to follow the old "multiply along the branches" of tree diagrams method and feel I cannot differienciate between when to use one method over another.

Any explanation of where I'm going wrong would be of much help, thank you very much.
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