Re: Help needed please

On 7 May 2009 13:34, DVH wrote in uk.politics.misc
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"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."


I think the OP didn't expect there are so many other factors to take into
account!

The OP is referring to an operation so it seems possible that his two
probabilities may not be independent of one another.

In practise, probabilities given to patients might be an informal manner
of expressing different degrees of risk (rather than a numerical measure
of risk):

"One percent" being a way of saying "not at all likely"
"Five percent" being a way of saying "unlikely but not to be ignored".

OTOH perhaps he posts those percentages as a way of referencing some hard
data about outcomes which he has come across but which he doesn't quote.
.

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