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RE: st: correlated data
"Nick Cox" <email@example.com>
RE: st: correlated data
Tue, 1 Sep 2009 19:06:42 +0100
I don't think anyone can guarantee that corresponding pixels (voxels) on
different images are measuring the same element of body, so what is to
be paired with what?
[mailto:firstname.lastname@example.org] On Behalf Of Richard
Sent: 01 September 2009 14:24
Subject: Re: st: correlated data
To my mind the scanned raw data signal is the variable (as per BP or
The instrument that performs the measurement (software) is the
measurement technique under examination and measures the raw data with
error In this case the old standard vs 3 new techniques
I don't see the correlation problem, isn't it one of assessing
agreement on a continuous scale & which technique is least biased?
You could perform analyses of old standard vs each of the new
I believe that OLS regression is not appropriate as both y (old
technique) & x (new technique) variables are both measured with error
Approaches suggested include least products regression or the Bland
Altman method of differences
this reference may be helpful: Ludbrook, J. Statistical techniques
for comparing measures and methods of measurement: a critical review.
Clinical and Experimental Pharmacology and Physiology. 2002;29:527 -
Cheers Richard Hiscock
On 1 Sep 2009, at 13:33, Nikolaos Pandis wrote:
> Hi to all.
> I would like to ask the following question.
> We have a set of 3-D images constructed from cat scans, and we are
> measuring volumes defined by certain anatomical points on the 3-D
> The reconstruction/measuring technique is performed using 3 new
> types of software and their results will be compared with the
> results of validated/reference technique.
> The same reconstructions/cat scans are used for all techniques.
> The objective is to see how close (do they differ significantly?)
> the volume values recorded by each technique are to the values
> recorded by the reference technique.
> I was thinking along the lines of regression with the
> volume(continuous) variable as the dependent variable and technique
> as the categorical dependent variable with 4 levels. The reference
> level would be the the standard/validated method.
> However, how would I account for the fact that the data is
> correlated since all measurements for the 4 methods are taken from
> the same reconstructions/scans?
> Any suggestions would be greatly appreciated.
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