On Nov 1, 7:32 pm, Ian wrote:
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.

I appologise for starting a statistical discussion; there's enough of
that at work. The important point is the one made in Chris's post
that gliding carries a risk in the range of one to between two and
three thousand depending on what denominator is used. Serving in Iraq
carries a similar risk, but the population isn't uniform; there's
clearly a difference in being in being part of the rear echelon vs
patrolling the streets. Our population is similarly stratified in
terms of risk.

There are also, unfortunately, factors beyond personal control in
play. As I've aged, I've noted that I need a bit more distance
between myself and the car in front of me. My head needs to swivel to
a greater degree to see things in the periphery. These effects can be
measured/quantified. Medical events such as stroke or MI have been
suggested as the cause of a number of accidents; the ones that killed
the student who soloed ahead of me where I learned to fly in SoCal and
that of a friend who crashed during the return to Kitty Hawk were
likely caused by medical incapacitation. As I age, my risk of such
events increases.

The important thing, IMHO, is to recognize the risk, decide what
you're willing to accept, and, if you choose to go forward, control
what you can and hope that what you can't won't kill you.

Ray Warshaw
1LK


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