On Tue, 18 May 2004 02:47:10 GMT, David Winsemius <[EMAIL PROTECTED]> wrote:
>Richard Hoenes wrote in news:[EMAIL PROTECTED]: > >> On Sat, 15 May 2004 19:42:42 GMT, David Winsemius >> <[EMAIL PROTECTED]> wrote: >> >>>Richard Hoenes wrote in >>>news:[EMAIL PROTECTED]: >>> >>>> >>>>>> If it was the ICC he was pushing I wouldn't mind so much, but he >>>>>> has insisted we include Bland & Altman's limits of agreement >>>>>> (which is simply the mean difference +/- [1.96*stddev] which has >>>>>> no signficance test), and he is now systematically having us >>>>>> remove every other statistical test we've included in the paper. >>>>>> The only other test left in the paper is the paired t-test and now >>>>>> he wants a reference >>>to >>>>>> show it is valid to use. >snipped stuff... > > The paired t-test >>>will be significant at the 95% level in exactly those situations when >>>the mean is more than t(0.025,n-1) or t(0.975,n-1) standard deviations >>>away from zero. t(0.025, n) will generally be greater than 1.96 but by >>>not much. Reporting the confidence interval is more informative than >>>merely reporting that the test was "significant", because it defines a >>>range of plausible values for the difference. >> >> We did this when we were told to put in the Bland Altman since we >> understood it more and, at least to us, it was more widely used. We >> were told to take it out of the paper since we'll have the Bland >> Altman intervals. > >I just looked at the Bland-Altman paper. In their example on p5 they did >precisely a paired t-test with the use of the correct t-level based on the >number of subjects. I don't know who corrupted that to the mean +/- >1.96sd's but I am sure Bland or Altman wouldn't tell you to use 1.96 in a >study with small numbers. http://www.mbland.sghms.ac.uk/ba.htm > > >Below is reference that is a direct response to your original request for a >citation in support of using paired t-test for measuring concordance. But >since Bland And Altman used the paired-t, too, I cannot see that it is >really necessary. Just use page 5 of B-A. Other tests of agreement are also >described. It describes the Lin concordance coefficient (AKA CCC, >concordance correlation coefficient) which has features similar to a >correlation coefficient but will be sensitive to either a difference in >means or a difference in standard deviations. Unfortunately the confidence >interval for this coefficient is given in another citation. Another >possible problem arises because I found a citation saying that it was >equivalent to the ICC. (Carrasco and Jover, Biometrics, v 59 n4) > >http://www3.oup.co.uk/eortho/hdb/Volume_22/Issue_03/pdf/220257.pdf >> >>>You could use Rosner's "Fundamentals of Biostatistics" or most basic >>>stats books for this assertion. The accept-reject formalism and the >>>confidence interval formalism have been shown to be equivalent, oh, >>>about a half century ago. >>> >>>If you want to "bring back the Pearson's correlation", why don't you >>>instead create a scatterplot of subject pre-post measures. >snipped more stuff >> The test we are reporting on is actually a series of vision tests with >> different scales, so this would entail a large number of plots. We >> were told not to include Bland Altman plots because of the large >> number, so scatterplots wouldn't work. >> >You could report summary estimate on a large number of groups that coincide >with the Bland-Altman framework by calculating the regression coefficients >of X-Y against Mean(X,Y). If you centered the mean, you could also look at >the intercept and s.e as a test of drift. The test for agreement would be >looking at the slope of the line, its standard eror, and seeing if it was >or wasn't plausibly different than zero. The usual caveats about >insensitivity to curvilinear departures apply. > >This all assumes that there was no intervention between the measurements. >This is a test for agreement where agreement should exist, right? No >interventions or expectation of drift? That is correct. Thanks for the references and info. . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
