From: hes@unity.ncsu.edu (Henry E. Schaffer)
Subject: Re: reduction of availability
Summary: Long boring discussion of basic statistical interpretation.
Organization: North Carolina State University
Date: Thu, 24 Dec 1992 16:57:14 GMT

In article <1hc9loINNaqk@usenet.INS.CWRU.Edu> bz269@cleveland.Freenet.Edu (James D. Del Vecchio) writes:
> ...
>	_If_ the list were made complete (re above note), and 
>it still bore out the same trend, then yes, it would show a 
>correlation.  Correlation != causation, of course. 
               ^^^^^^^^^^^^^^^^^^^^^^^^
  Of course!
>
>If it showed a before and after comparison for each country re
>the onset of their gun laws and how their murder rate changes
>then one could point to some causation.

  If one assumed that nothing else changed.  This is a *very*
big assumption, and is at the root of the subject called
"experimental design."  The problem is that there is no
"experimental control" used, nor can there be a good experimental
control.  (E.g., take two identical countries, institute gun
control in one but not in the other, treat them identically except
for this, and watch the relative changes in murder rates.  Then
replicate this experiment N times, an analyze statistically 
whether or not there is a statistically significant difference in
murder rates between the two "treatments".  Also later do the
same kind of experiment but remove gun control in one of a pair
of identical countries, ...  Cf. "cross-over" experimental design.)

  The problem is that if gun-control is instituted that the
outcome (with respect to murder rates (M.R.)) is thoroughly ambigious.
Several possibilities:

M.R. goes down:  Possible interpretations: 1) gun control lowers
the M.R.; 2) the M.R. would have gone down anyway, as a result of other
factors without the institution of gun control - so gun control has
no effect; 3) the M.R. would have gone down even further if gun 
control hadn't been instituted - so gun control increases murder.

M.R. stays the same:  Possible interpretations: 1) gun control has
no effect on the M.R.; 2) the M.R. would have gone up if gun control
hadn't been instituted - so gun control works; 3) the M.R. would have
gone down even further if gun control hadn't been instituted - so gun 
control increases murder.

M.R. goes up:  Possible interpretations: 2) gun control increases
murder; 2) the M.R. would have gone even further up if gun control
hadn't been instituted - so gun control works; 3) gun control has no
effect on murder.

  But even these possibilities aren't sufficient - since one can 
also look at the M.R. trends before and after the institution of
gun control, with further additional interpretations.  Additionally
one can look at the year-to-year variation and either infer treands
from them or include/exclude years as being "non-representative" or
whatever.  In a "stable" situation there still will be year to year
variation, and so in any given year the next year has a 50% chance
of being higher (or lower)

  The ambiguity of the situation gives lots of room for people 
to grind their axes, and so I doubt that we'll ever run out of
postings in t.p.g.

--henry schaffer
Newsgroups: talk.politics.guns