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Re: st: Predicting sdres in Stata


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Predicting sdres in Stata
Date   Fri, 22 Jul 2011 09:19:36 -0500

A footnote to Paul's excellent suggestions. Bland-Altman plot is a
common biostatistician's name for plotting the difference between two
variables versus their sum (or equivalently their mean), a plot which
long predates Bland and Altman, although that in no sense detracts
from the usefulness of their papers. Other Stata implementations
include -pairplot- (SSC) and -concord- (SJ). Further references can be
found in or via the help for those programs.

On Fri, Jul 22, 2011 at 3:29 AM, Seed, Paul <paul.seed@kcl.ac.uk> wrote:

> You say you have two groups of subjects; but your code suggests that
> you have measurements by two different methods on one group of subjects.
>
> The issues are totally different in the two cases.
>
> In the first case, you can use multiple regression:
> All you need is:
>        regress depenVar BSA group, vce(robust)
>
> In the second case, you are comparing methods of measurement, not
> groups of subjects.  The BSA scores are irrelevant, because
> they are the same for each pair of measurements.
>
> i)  You would be well advised to read Bland & Altman's classic
> paper: Bland, J. Martin and Altman, Douglas G.  Statistical
> methods for assessing agreement between two methods of clinical
> measurement. Lancet, 1986; i: 307-310.  A Google search should
> turn up a copy.
>
> ii) Read it again.  It really is that good.
>
> iii) The simplest way to perform Bland-Altman plots in Stata is
> by using the user-written command -baplot-
>
> Type
>        findit baplot
>
> and follow the blue links. That should place a copy
> ready to use on your computer.  You do not need administrator rights.
> Then type
>        help baplot
> to find out how to use it.
>
>        baplot depenVar1 depenVar2
>
> There is an alternative, -batplot-, which should be used with the
> -notrend- option:
>
>        batplot depenVar1 depenVar2, notrend
>
>
>
> Paul T Seed, Senior Lecturer in Medical Statistics,
> Division of Women's Health, King's College London and King's Health Partners
> 020 7188 3642.
>
>
>
>
>
>
> -----Original Message-----
> From: Lars Folkestad [mailto:lfolkestad@health.sdu.dk]
> Sent: 21 July 2011 10:48
> To: Seed, Paul
> Subject: Re: st: Predicting sdres in Stata

>
> When i predict adjdepenVar1 using the xb option post regression, i am
> getting the BSA adjusted variable right?
>
> My problem is: I want to ensure that the difference in BMD between two
> groups is not due to BSA difference alone.
>
> Doing the regression and predicting an adjustedBMD variable and then doing a
> ttest for between group differences and getting a p-value of below 0.05 will
> render this statement true, the differences between the two groups are NOT
> due to the differences in BSA alone.
>

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