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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: net.aviation
Subject: Pilot Proficiency Program
Message-ID: <3837@alice.UUCP>
Date: Fri, 7-Jun-85 16:22:50 EDT
Article-I.D.: alice.3837
Posted: Fri Jun  7 16:22:50 1985
Date-Received: Sat, 8-Jun-85 03:49:58 EDT
References: <799@ihnp4.UUCP>
Organization: Bell Labs, Murray Hill
Lines: 18

Jeff Williams made the following comment in another context:

	4.  Annual participation in the Pilot Proficiency Program -
		the record speaks for itself as to the decrease in
		accidents for pilots who participate

While I don't doubt for an instant that participation in
the Pilot Proficiency Program has a strong negative correlation
to accident rates, that does not mean that there is a direct
causal relationship!

Putting it differently: suppose we know that people who participate
in PPP stay out of trouble.  Mightn't it be that the sort of
person who participates in PPP is also the sort of person who
knows enough not to do stupid things?

One must be very careful about trying to draw statistical
conclusions about a self-selected sample.