Ecolog:
For consideration and discussion:
"The method of science . . . may be summarized . . . by saying that in the
scientific subculture, expectations are deliberately created by necessary
inferences from theoretical models, within the range of observations
suitable to the delicacy of the [enlarged] apparatus of the human senses. If
these expectations are disappointed then the images or models upon which
they are based must be reorganized, as there is no possibility of denying
the inference and very little possibility of denying the messages which do
not conform to the expectations." --Kenneth Boulding
WT
----- Original Message -----
From: "Ned Dochtermann" <[email protected]>
To: <[email protected]>
Sent: Saturday, December 05, 2009 5:29 PM
Subject: Re: [ECOLOG-L] an example of a false correlation
In that case I'd argue that your use of path analysis was essentially
exploratory in nature and thus would carry with it the same sort of caveats
as
any exploratory analysis whether conducted using regression (e.g. step-wise
regression) or any other exploratory analysis.
Ned Dochtermann
--
Ned Dochtermann
Department of Biology
University of Nevada, Reno
775-784-6781
[email protected]
www.unr.nevada.edu/~dochterm/
--
Quoting William Silvert <[email protected]>:
It is a long time since I used any kind of standard statistical analysis,
including Path Analysis, so I may be a bit rusty on this, but I do not
think
that the inference of causality is necessarily based on a priori
assumptions. In one case (Silvert, William. 1982. Top-down modeling of
multispecies fisheries. In "Multispecies Approaches to Fisheries
Management
Advice," M. C. Mercer, ed. Can. Spec. Publ. Fish. Aquat. Sci. 59:24-27.)
where I was investigating the role of environmental factors in fisheries I
tested a model based on the hypothesis that both effort and temperature
affected fish catches. But something seemed wrong, and when I tackled the
problem with Path Analysis I came up with the surprising result that catch
determined effort, and not the other way around. This seemed like
nonsense,
but I happened to meet the authors of the data report I was using, and
they
were greatly upset that I was analysing their data. Finally they confessed
that they had been told to collect effort data, but since they were unable
to do this they made up the numbers by assuming that whatever fish a boat
caught most of must have been what they were fishing for. Thus Path
Analysis
led me kicking and screaming to the right result - catch really did
determine effort!
Bill Silvert
----- Original Message -----
From: "Ned Dochtermann" <[email protected]>
To: "William Silvert" <[email protected]>
Cc: <[email protected]>
Sent: sábado, 5 de Dezembro de 2009 20:04
Subject: Re: [ECOLOG-L] an example of a false correlation
> It is interesting that so many researchers bring up the issue of
> "correlation
> doesn't demonstrate causality" and yet are more than happy to argue
> causation
> based on regression, which usually relies on the same sort of data as
> used
> for
> standard correlational analyses.
>
> Regression analyses are not typically used for experimental data and
> thus
> are
> asserting causation based on correlation. The practical difference
> between
> regression and correlation is the a priori hypothesizing of causation
> (how
> many
> regressions are really conducted post hoc after examining scatter plots
> is
> a
> whole other can of worms). The exact same statistical results in terms
> of
> "F's", "P's" and R^2 will be obtained if you switch which variables are
> dependent and independent in a linear analysis (of course intercepts and
> coefficients would differ).
>
> I mention this in relation to the below post because when discussing
> path
> analysis with a colleague awhile back the comment was made that
> "correlation=/=causation" to which my reply was "that's what you do with
> regression". Although path analysis does use correlation matrices (or
> covariance matrices) in its analysis I consider it to be more akin to
> multiple
> regression as you're testing a priori causal hypotheses.
>
> Next time someone asserts correlation doesn't demonstrate causation ask
> them
> when the last time was that they used regression. The ensuing argument
> can
> be
> quite amusing.
>
> Ned Dochtermann
>
> --
> Ned Dochtermann
> Department of Biology
> University of Nevada, Reno
>
> 775-784-6781
> [email protected]
> www.unr.nevada.edu/~dochterm/
> --
>
>
>
> Quoting William Silvert <[email protected]>:
>
>> Since I think that Mao Tse Tung and Pol Pot were heavy smokers, to say
>> nothing of Fidel Castro, I guess that where Malcolm is going is to show
>> that
>> by selecting your data you can obtain any correlation you want.
>>
>> The point that correlation alone does not prove causality is one that
>> statisticians are always making, but mention should be made of Path
>> Analysis, a technique based on multiple correlation, that is effective
>> in
>> establishing causal pathways. It is used extensively in the social
>> sciences,
>> but only rarely in ecology. I think that it deserves more attention, I
>> have
>> certainly found it of value.
>>
>> However a benthic ecologist I know who used Path Analysis died young,
>> so
>> although that is only one data point, perhaps Path Analysis should be
>> avoided!
>>
>> Bill Silvert
>>
>> ----- Original Message -----
>> From: "malcolm McCallum" <[email protected]>
>> To: <[email protected]>
>> Sent: sexta-feira, 4 de Dezembro de 2009 18:13
>> Subject: [ECOLOG-L] an example of a false correlation
>>
>>
>> > Anyone who teaches stats might be interested in this false
>> > correlation
>> > I
>> > just stumbled on.
>> >
>> > 1) Stalin did not smoke
>> > 2) Hitler did not smoke
>> > 3) Mussolini (sp?) did not smoke.
>> > 4) Roosevelt smoked
>> > 5) Churchill smoked
>> >
>> > I guess you can see where I'm going with that!
>> > :)
>> >
>> > --
>> > Malcolm L. McCallum
>>
--------------------------------------------------------------------------------
No virus found in this incoming message.
Checked by AVG - www.avg.com
Version: 8.5.426 / Virus Database: 270.14.95/2546 - Release Date: 12/05/09
08:13:00
