Many thanks to all those who responded to my query below....

>When you are making a comparison between two different methods used to measure
the same thing the aim is to assess >their agreement with one another. For
example, you might want to compare measurements made by a current piece of
>equipment with measurements made by a new piece of equipment (but the true
measurement is not known). Using simple >correlation to look at the relationship
is not the right thing to do since, amongst other reasons, you would expect
there to be >quite a high degree of correlation between two methods which were,
after all, designed to measure the same thing!
>
>In the past I have always used plots of mean value against the difference
(sometimes known as Bland and Altman plots in >certain circles!) as described in
Bland & Altman's paper in the Lancet (1986) which also includes an excellent
explanantion >of why correlation is not a suitable method to assess agreement
with!
>However, I have recently come across something called the coefficient of
concordance (in the book 'Biostatistical >Analysis' by Zar) and wondered if
anyone has any opinions on experience of use or comparison of methods or knows
of >any other methods used to assess agreement of this type that they would like
to share with me! There doesn't seem to be a >great deal of readily accessible
information around on this subject.

Lots of different methods were suggested along with some interesting opinions.
In addition to Lin's coefficient of concordance and the limits of agreement
method by Bland and Altman, the other methods that were suggested were:
Kappa statistic (although this is for categorical data not continuous)
Multitrait-Multimethod model (MTMM)gage R&R analysis
Data envelopment analysis
Passing-Bablok regression

References suggested were:

Measurement in Medicine by Ludbrook (1997) 24(2) 193-203

Mandel, K and Stiehler, R D. "Sensitivity - A criterion for the comparison of
methods test" J. Res. Natl. Bur. Stand. 1954; 53(3):155-159

Tan, C Y and Iglewicz, B. "Measurement - methods comparisons and linear
statistical relationship" Technometrics. 1999; 41(3):192-201

Bartko (1994) Measures of agreement: a single procedure. Statistics and medicine

Lin (1989) A concordance correlation coefficient to evaluate reproducibility.
Biometrics (1989):45:255-268

Martin R F. General Deming regression for estimating systematic bias and its
confidence interval in method-comparing studies. Clinical Chemistry;
46(1):100-104 (2000)

Bland JM, ALtman DG. Measuring agreement in measurement comparison studies.
Statistical Methods in Medical Research (1999);8:135-160

Morton AP, Dobson AJ. Assessing sgreement. Medical Journal of Australia (1989);
150:384-387

"Evaluatng the Measurement Process" by Donald J Wheeler and Richard W Lyday. SPC
Press Inc.

Passing H, Bablok W: A new biometrical procedure for testing the equality of
measurements from two different analytical methods. J Clin. Chem. Clin. Biochem.
(1983);21:709-720

Passing H, Bablok, W.: Comparison of several regression procedures for method
comparison studies and determination of sample sizes. J Clin. Chem. Clin.
Biochem. (1984);22:431-445

Dhanoa, MS et al. Use of mean square prediction error analysis and
reproducibility measures to study Near Infrared calibration equatrion
performance. Journal of Near Infrared Spectroscopy:7:133-143

The most noteable comment made was probably that by Doug Altman...that Lin's
coefficient of concordance is a measure of *relative* agreement whereas the
limits of agreement method proposed by Bland and Altman assess *absolute*
agreement.

If anyone is interested in the replies in more detail please contact me (not the
list!!!) and I will be happy to forward this on as an appropriate file.

Thanks again to all those who replied,

JOY

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