Many years ago now, the FDA discovered that with the advent of their new
computer system they could do quick retrospective studies of various drugs
to see which were most closely associated with death of the patient. Which
drugs were the most dangerous. The first study they undertook looked at
correlations between a large variety of drugs and sudden death in the
operating room. The data were entered, the numbers crunched, and it was
discovered (surprise!) that the most dangerous drug, with nearly a 100%
correlation with sudden death in the operating room was (wait for it... )
Oxygen. Virtually every patient who died in the operating room had received
oxygen.

Correlations are not necessarily cause and effect. Also, be very careful
that your study is designed to tell you what you want to find out.

--
Bob (Chief Pilot, White Knuckle Airways)

I don't have to like Bush and Cheney (Or Kerry, for that matter) to love
America

"Veeduber" wrote in message
...


Back when the world was young and I still had hair the Navy hired a gaggle
of
eggheads to contribute to the design of what eventually became the
Spruance-class of destroyers (i.e., DD-963 class). At that time I was the
Leading Chief of the computer shop for Pac Fleet's cruiser-destroyer
force. I
was told to give the eggheads access to anything they wanted in the way of
maintenance and repair data, which I did with a cheery aye-aye, sir.

Marvelous stuff, watching those eggheads at work, doing their computerized
statistical analysis of equipment failures, tracking everything back to
the
manufacturer on one hand and the Navy schools on the other.

The product of their work was a list of recommended equipment to go into
the
new ships; only the best stuff as determined by its failure rate, required
maintenance man-hours, mean time to repair and so forth.

Which was all bull****, unfortunately.

At that time (early 1970's) ComCruDesPac had about 137 ships. The
analysis
covered such things as electric motors, pumps, air compressors, ammo
hoists and
so forth, the ancillary systems that are the glue of a modern-day
warship.
(The hull design and the turbine powerplants were determined by other
groups.)
The objective of the study was to determine the best of that equipment and
on
the surface, their methods of analysis appeared valid. But in providing
them
with data I noticed that while all destroyers had high-pressure air
compressors
(for example) some of them had never failed. (Not many... four, I think.)
Same thing for the other components. All of the ships used a certain type
of
gear-head motors but a few ships had never reported any problems with
them.
Which brings up a point worthy of mention.

Even though built to the same plan, vessels within a given class are not
identical. The ships are built at different yards and while their specs
were
identical their equipment came from a variety of manufacturers. In the
case of
electric motors for example, while most of the ships used motors from
General
Electric or Westinghouse a few of them had motors from manufacturers I'd
never
heard of. The key point here is that some ships had never reported any
form of
failure for certain pieces of equipment.

The bottom line is that the study failed to consider the possibility that
some
equipment had never failed. Their final report identified only equipment
that
HAD failed, giving high marks for designs and manufacturers that failed
the
least often.

Which completely ignored the Really Good Stuff.

- - - - - - - - - - - - - - - - - - - - - - - - - - - -

So what's all that got to do with airplanes? Quite a bit, when it comes
to
home-builts.

A fairly common thread on various aviation-related mailing lists and
newsgroups
is someone polling the subscribers in hopes of determining the 'best'
...whatever. The best way to paint a spam can; the best brand of tire;
the
best vacuum pump and so on. Which gets down-right scary at times. (One
such
poll decided that the 'best' aluminum was 6061 :-)

Polls and surveys, and the methods of statistical analysis that supports
them,
are valid tools. But only when your sample is an accurate reflection of
the
population being polled. Ask a room-full of pre-schoolers to define a
balanced
diet, don't be surprised if the answer is graham crackers and milk. In a
similar vein, wood makes the best fuselage (according to builders of
Pietenpol
'Air Campers'), welding is easy (according to experienced weldors) and
flying is inexpensive (according to people earning $100k p/a or more). In
the
case of the New Ship Design Study Group they failed to include the entire
population of ancillary equipment, inadvertently limiting their
investigation
to equipment having a history of failure.

SO WHAT'S THE POINT?

The Internet offers unprecedented access to information but does not
provide
any means of determining if that information is valid. Indeed, within the
field of home-built aviation only a small percentage - - probably less
than
five percent - - of the available information is valid and even then, only
in a
particular case. The remainder is either skewed by commercial interest or
is a
reflection of 'conventional wisdom,' wherein the poster is simply
parroting
something they have heard.

Common sense has become remarkably uncommon stuff in modern-day America.
Given
the risk inherent in rising above the ground on wings I believe the wiser
course is to treat ALL information on the internet as invalid until you
can
test it yourself. Fortunately, with a technical subject such as aviation
the
required tests are fundamental and well defined. For the homebuilder,
especially those lacking an engineering background, the tricky bit is
devising
methods of applying such tests to their particular situation.

-R.S.Hoover

PS - - So what happened with regard to selecting failure-prone equipment?
I've
no idea. By the time the first of the new class slid down the ways I'd
been
retired for a number of years. But it's interesting to note that several
of
the Spruance-class have been scraped after barely twenty years service.
(Navy
ships are designed for a minimum service life of thirty years.)

I identified the Really Good Stuff aboard our own ships and submitted a
report
on the matter, producing a minor controversy with regard to maintenance.
Sailors know what I'm talking about and it really doesn't apply to anyone
else.