Math Is Fun Forum

eclipse4227

Member

Registered: 2007-04-23

Posts: 10

muliple probability

how do you calculate the probability of something happening when there are multiple percentages? i know you cant just add them haha
the problem: there is a 4% chance something won't happen. then there is a separate 9% chance it won't happen. again separate is a 7% chance it won't happen. what's the overall probability of the event not happening?

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: muliple probability

Hi eclipse;

  You would be able to add them if they are independent of each other. If that is what you mean by separate ( they are independent ) then just add. There is a formula for when they aren't independent but you haven't provided enough info for me to use it. You were probably given a Venn diagram, provide me with that info.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

eclipse4227

Member

Registered: 2007-04-23

Posts: 10

Re: muliple probability

No venn diagram. It's the mechanics of a game that im just curious about. Each stat is a separate occurrence. The stats I'm looking at give 4% chance to block an attack. If this fails you get another 9% chance to block. If this fails you then have a 7% chance to block. I've checked with adding them and it doesn't work that way because there is a spec that gives u +80% to block. This would put me at 100% chance to block but you can still occasionally get hit.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: muliple probability

Hi eclipse;

Looks like a binary tree so you multiply and add:

So there is almost a 19% chance that event won't happen.

or somewhat easier:

Last edited by bobbym (2009-04-25 18:38:38)

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: muliple probability

Like bobbym said, the quickest way to do this is to work out the chance that you do get hit, because that's just the chance that all three dodge tests are failures.

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Why did the vector cross the road?
It wanted to be normal.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: muliple probability

Hi mathsyperson;

  Thanks for checking my work, probability is so error prone and so counter intuitive.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

eclipse4227

Member

Registered: 2007-04-23

Posts: 10

Re: muliple probability

Ok. Thanks for the help.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: muliple probability

Your welcome.

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.