morphmet
Tue, 24 Nov 2009 12:51:22 -0800
-------- Original Message --------
Subject: RE: Sample size
Date: Tue, 24 Nov 2009 17:13:42 GMT
From: Samor Gandhi <samorgan...@yahoo.com>
To: morphmet@morphometrics.org
Thank you very much!
--- On *Tue, 11/17/09, morphmet
/<morphmet_modera...@morphometrics.org>/* wrote:
From: morphmet <morphmet_modera...@morphometrics.org>
Subject: RE: Sample size
To: "morphmet" <morphmet@morphometrics.org>
Date: Tuesday, November 17, 2009, 10:42 PM
Thought I posted this earlier. Apparently not. Sorry. -mod
-------- Original Message --------
Subject: RE: Sample size
Date: Fri, 13 Nov 2009 08:32:10 -0800 (PST)
From: F. James Rohlf <ro...@life.bio.sunysb.edu
</mc/compose?to=ro...@life.bio.sunysb.edu>>
Organization: Stony Brook University
To: morphmet@morphometrics.org
</mc/compose?to=morph...@morphometrics.org>
References: <4afd72f9.1040...@morphometrics.org
</mc/compose?to=4afd72f9.1040...@morphometrics.org>>
There is no rule of thumb. Unequal sample sizes are just not an
efficient use of one's efforts in making measurements.
For example, with two samples of sizes n1 and n2 and n1+n2 = 100 a
split
like 10 and 90 has an effective average sample size of just 18
(equivalent to a total of just 36 samples rather than the actual
100). A split of 2 and 98 has an effective average sample size of
just 3.92 (equivalent to a total sample size of just 5.92).
The same considerations apply to multivariate data except that one
must also be concerned with having sample sizes large enough so that
the covariance matrix is not singular (see earlier discussion on
morphmet).
----------------------
F. James Rohlf, Distinguished Professor
Dept. Ecology and Evolution, Stony Brook University, NY 11794-5245
-----Original Message-----
From: morphmet [mailto:morphmet_modera...@morphometrics.org
</mc/compose?to=morphmet_modera...@morphometrics.org>]
Sent: Friday, November 13, 2009 9:54 AM
To: morphmet
Subject: RE: Sample size
-------- Original Message --------
Subject: RE: Sample size
Date: Fri, 13 Nov 2009 04:34:31 -0800 (PST)
From: Samuel Okoye <samu...@yahoo.com
</mc/compose?to=samu...@yahoo.com>>
To: morphmet@morphometrics.org
</mc/compose?to=morph...@morphometrics.org>
Thank you very much for your reply.
If it is not required to have equal sample sizes (n1=n2), is there any
rule of thump how big the difference between the two group (e.g. n1=5
n2=15)? Does this also apply for multivariate data?
Many thaks in advance,
Samuel
--- On *Thu, 11/12/09, morphmet
/<morphmet_modera...@morphometrics.org
</mc/compose?to=morphmet_modera...@morphometrics.org>>/* wrote:
From: morphmet <morphmet_modera...@morphometrics.org
</mc/compose?to=morphmet_modera...@morphometrics.org>>
Subject: RE: Sample size
To: "morphmet" <morphmet@morphometrics.org
</mc/compose?to=morph...@morphometrics.org>>
Date: Thursday, November 12, 2009, 10:58 AM
-------- Original Message --------
Subject: RE: Sample size
Date: Thu, 12 Nov 2009 10:39:35 -0800 (PST)
From: F. James Rohlf <ro...@life.bio.sunysb.edu
</mc/compose?to=ro...@life.bio.sunysb.edu>
</mc/compose?to=ro...@life.bio.sunysb.edu
</mc/compose?to=ro...@life.bio.sunysb.edu>>>
Organization: Ecology & Evolution
To: <morphmet@morphometrics.org
</mc/compose?to=morph...@morphometrics.org>
</mc/compose?to=morph...@morphometrics.org
</mc/compose?to=morph...@morphometrics.org>>>
References: <4af5219d.1070...@morphometrics.org
</mc/compose?to=4af5219d.1070...@morphometrics.org>
</mc/compose?to=4af5219d.1070...@morphometrics.org
</mc/compose?to=4af5219d.1070...@morphometrics.org>>>
Do you mean for morphometric data (multivariate) or just
univariate
data? For the univariate case you could check Section 9.8 in
Biometry
for a method to estimate sample size. It does not, however,
consider
the unequal sample size case.
In general, equal sample sizes are not required. The main
advantage of
equal sample sizes is that they are more efficient. For a given
total
sample size, n1+n2, you will have greater statistical power if
n1=n2.
Having equal sample sizes also make the computations slightly
simpler
but that is no longer much of an issue when computers are used to
perform all of the calculations.
=========================
F. James Rohlf
Distinguished Professor, Stony Brook University
http://life.bio.sunysb.edu/ee/rohlf
> -----Original Message-----
> From: morphmet [mailto:morphmet_modera...@morphometrics.org
</mc/compose?to=morphmet_modera...@morphometrics.org>
</mc/compose?to=morphmet_modera...@morphometrics.org
</mc/compose?to=morphmet_modera...@morphometrics.org>>]
> Sent: Saturday, November 07, 2009 2:28 AM
> To: morphmet
> Subject: Sample size
>
>
>
> -------- Original Message --------
> Subject: Sample size
> Date: Wed, 4 Nov 2009 06:40:15 -0800 (PST)
> From: Samuel Okoye <samu...@yahoo.com
</mc/compose?to=samu...@yahoo.com>
</mc/compose?to=samu...@yahoo.com
</mc/compose?to=samu...@yahoo.com>>>
> To: morphmet_modera...@morphometrics.org
</mc/compose?to=morphmet_modera...@morphometrics.org>
</mc/compose?to=morphmet_modera...@morphometrics.org
</mc/compose?to=morphmet_modera...@morphometrics.org>>
>
>
>
> Dear all,
>
> I would be most grateful if you tell me how important to have
> balanced
> sample sizes for the t-test and ANOVA? Should the sample sizes
> always be
> equal? If not how big should the difference between them
be? How
> can I
> do sample size calculaction for unbalanced t-test or ANOVA?
>
> Many thanks in advance,
> Samuel
>
>
>
> --
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