On 1 Nov, 22:46, Chris Reed wrote:

If we ignore the past, however, each day's chance is the same at 0.5.
Thus Ray (may he live forever) is able to state that next year his
chances will be pretty much the same, if he makes it that far.

That's not his claim, though. He seems to be saying that his chance of
dying tomorrow is 1 in 80, his chance of dying the day after that is 1
in 80 and so on to 1st November 2008, but that his chance of dying at
all during that year is still only 1 in 80. I do hope he'll explain,
in case I'm missing something obvious ...

Cumulation of probabilities is what the human brain does automatically.
Suppose the chance of being killed on a glider flight is 1 in 1,000. The
mind (without extensive training) deals with this in a number of ways:

The prblem, of course, is that one cannot be killed twice, so it does
not make sense to combine (simply) the probabilities of dying on
different days. Multiple survival is the aim, and survival
probabilities can be combined quite easily. Just recast our statement
as "The chance of surviving a glider flight is 999 in 1,000"...

3. At my club we fly 1,000 flights a year between us, so one of us is
sure to die flying.

There's actually a 63% chance that one of you will have bought the
farm by the end of the year. Get a new safety officer!

a. In the UK where I fly, gliding fatalities are on average around 2.5
per annum out of 5,000 pilots, so my "statistical" risk is around 1 in
2,000 of dying through gliding each year.

Agreed.

b. I can do a number of things to reduce my personal risk to less than 1
in 2,000, so I'll try to do those things.

Agreed.

c. This is, to me, an acceptable level of risk for the pleasure I get
from gliding.

Agreed.

The good thing is that these probabilities are not cumulative. I've been
flying for 11 years, so if they were cumulative my "statistical" risk
might be down to under 1 in 20. It ain't.

Your chance of making it through the next 11 years is still 99.5% I
reckon you can improve that to 99.95% by application of non-stupidity!

Ian