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# Re: st: RE: comparison of agreement plot for non-Normal data

 From Steve Samuels To statalist@hsphsun2.harvard.edu Subject Re: st: RE: comparison of agreement plot for non-Normal data Date Thu, 8 Apr 2010 10:44:21 -0400

```"This does not produce the same 95% CI as the equation from the double
regression (recommended in the paper mentioned above)"
b0 + b1A ą 1.96 * residual SD from the regression."
I assume you mean:
b0 + b1A +/ 1.96 * residual SD

This is not the equation for a confidence interval.  The variability
of a predicted mean depends on the distance of the predictor variable
(A) from the sample mean. .  See the correct equation in the section
on linear regression of any introductory text.

Secondarily,  -twoway lfitci- will use a t-multiplier not a z-
(Gaussian) multiplier

Steve

> Pinto, Daniel
>
> I am analyzing the results of a method comparison study assessing agreement between two methods of capturing health service use and costs, N=50. Due to the small sample size and the analysis of cost data my distribution is non-Normal. I believe that Bland-Altman plot is the best statistic to use except that it assumes a Normal distribution. To address non-Normality Bland and Altman recommend using a double-regression half-Normal distribution method in the following paper: Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res. 1999 Jun;8(2):135-60.
>
> I have attempted the performance of this method in STATA 10, plotting the residuals of the difference in GP count against the mean of the GP count.
>
> regress gpcntdif meangpcnt
>
> predict gpcntdifresid, resid
>
> regress gpcntdifresid meangpcnt
>
> I have tried to produce the plot including 95% CI using: twoway lfitci gpcntdifresid meangpcnt, stdf || scatter gpcntdifresid meangpcnt
>
> This does not produce the same 95% CI as the equation from the double regression (recommended in the paper mentioned above):
> b0 + b1A ą 1.96 * residual SD from the regression.
>
> Although I can calculate the 95% CI using the equation I am unable to apply the 95% CI lines to the B-A plot. Is there anyway how to do this?
>
>
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>

--
Steven Samuels
sjsamuels@gmail.com
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```