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Re: Experiments with large N
Massimo, thank you for this very interesting reference. I think the
point here is not so much about large N per se, but about whether one is
interested in characterizing the typical population (for which a more
limited sample is sufficient), or whether one is interested in
investigating the extremes of a population, which by definition will
require one to go and look for the tails of the distribution. Semal and
Demany did a smart thing: they pretested 68 people (not such a huge
number to test, actually, especially in a group setting as was done
here), and then did more intensive testing with a small group of people
(N=9) who specifically did poorly on the initial screening. This a good
way to proceed if you want to find the extremes of a population.
Psychoacousticians tend to use a small number of highly trained
listeners, but many labs including my own, do not follow this convention
unless we are specifically interested in testing the effects of
training. (Indeed, this sometimes gets us into trouble with reviewers
who insist that we should only use experienced listeners or else the
data are too noisy and not representative--actually it can be the
opposite, in some instances, since training and experience have their
Robert J. Zatorre, Ph.D.
Montreal Neurological Institute
3801 University St.
Montreal, QC Canada H3A 2B4
web site: www.zlab.mcgill.ca
Massimo Grassi wrote:
Huge samples are very nice if you can get 'em, though such is not
always the case, alas.
If I had a large (very large) N I would be more interested in have a
glance of the population distribution. And this is the reason why, I
think, large N studies are important.
For example, last year Semal and Demany (JASA, 2006) showed that there
are listerers that are able to detect subtle pitch differences but, at
the same time, are not able to tell the direction of these pitch
differences (up or down). They showed also that there are listeners that
do not show such a dissociation. How is the distribution of this trait
among the population? Are the first listeners a small or a relatively
large amount of the population?
Think about. Pitch perception is one of the most investigated topic in
auditory perception. Why do we discover this only in 2006? Is it because
the average study is run with few listeners only and these listeners
are, in the majority of cases, the "usual suspects" (e.g., authors,
people with a background of a minimum of 1000 hours of psycho-beeps
In textbooks we often read that JND for pitch is about 1%. After reading
Semal and Demany results I asked myself: is this figure real? Does it
reflect all the population? Which part of the population? For which
It is trivial to stress that, with large N, statistical power goes soon
very high. I think the goal of large N study is different: have, at
least once, a real glance of the population.