Robert J. MacG. Dawson wrote:
> [EMAIL PROTECTED] wrote:
> >
> > In article <[EMAIL PROTECTED]>,
> > Jerry Dallal <[EMAIL PROTECTED]> wrote:
> >
> > > (1) statistical significance usually is unrelated to practice
> > > importance.
> >
> > I don't think so. I can think of many examples in which statistical
> > inference plays an invaluable role in practical applications and
> > instrumentation, or indeed any "practical" application of a theory etc.
> > Not just in science, but engineering, e.g aircraft design, studying the
> > brain, electrical enginerring. Certainly there are examples of
> > statistical nonsense, e.g. polls, but i wouldn't go so far as to say it
> > is usually like this.
>
> Chris: That's not what Jerry means. What he's saying is that if your
> sample size is large enough, a difference may be statistically
> significant (a term which has a very precise meaning, especially to the
> Apostles of the Holy 5%) but not large enough to be practically
> important. [A hypothetical very large sample might show, let us say,
> that a very expensive diet supplement reduced one's chances of a heart
> attack by 1/10 of 1%.] Alternatively, in an imperfectly-controlled
> study, it may show an effect that - whether large enough to be of
> interest or not - is too small to ascribe a cause to. [A moderately
> large study might show that some ethnic group has a 1% higher rate of
> heart attacks, with amargin of error of +- .2% . But we might have, for
> an effect of this size, no way of telling whether it's due to genes,
> diet, socioeconomic factors, recreational drugs, or whatever.]
I'd add that I think Jerry meant "unrelated" in the sense of independent rather
than irrelevant (Jerry can correct me if I'm wrong). You can get important
significant effects, unimportant significant effects, important non-significant
effects and unimportant non-significant effects.
For what its worth, practical importantance also depends on many factors other
than effect size. These include mutability, generalizabilty, cost, and so on.
Thom
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