Re: Help needed please


"True Blue" <tb@xxxxxxxxx> wrote in message
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"DVH" <dvh@xxxxxxxxxxxx> wrote in message
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"Robert Henderson" <philip@xxxxxxxxxxxxxxxxxxxx> wrote in message
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Suppose a process, say an operation, has a 5% chance of one thing
happening and a !% chance of another thing happening and both happen
what is the probability of both happening?

Please show your workings if you can supply me with the answer.

You multiply the two probabilities.

Probability is expressed as a figure P between 0 and 1.

Your first thing has a P of 0.05

Assuming the exclamation mark in your post is meant to be the figure one,
your second thing has a P of 0.01

P(A) x P(B) = 0.05 x 0.01 = 0.0005

Expressed as a percentage, that's 0.05%.


Now, suppose the surgeon says: "Mr Henderson, there is a 5% probability we
will need to circumcise you and a 1% probability that we will need to
amputate your penis". The second case *by extension* encapsulates the
first. Do you therefore simply add the probabilities together?

:-)

Apparently in real life you need to cluster probability.

"To understand the mathematics of correlation better, consider something
simple, like a kid in an elementary school: Let's call her Alice. The
probability that her parents will get divorced this year is about 5 percent,
the risk of her getting head lice is about 5 percent, the chance of her
seeing a teacher slip on a banana peel is about 5 percent, and the
likelihood of her winning the class spelling bee is about 5 percent. If
investors were trading securities based on the chances of those things
happening only to Alice, they would all trade at more or less the same
price.

But something important happens when we start looking at two kids rather
than one-not just Alice but also the girl she sits next to, Britney. If
Britney's parents get divorced, what are the chances that Alice's parents
will get divorced, too? Still about 5 percent: The correlation there is
close to zero. But if Britney gets head lice, the chance that Alice will get
head lice is much higher, about 50 percent-which means the correlation is
probably up in the 0.5 range. If Britney sees a teacher slip on a banana
peel, what is the chance that Alice will see it, too? Very high indeed,
since they sit next to each other: It could be as much as 95 percent, which
means the correlation is close to 1. And if Britney wins the class spelling
bee, the chance of Alice winning it is zero, which means the correlation is
negative: -1."


.

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