Thank you, David, for the answer. It helps a little, but I have some
further questions about -xtsum-. I wish, there were more details in
manual.
I need the varians components to calculate the ICC (intra class
correlation). I tried different methods to calculate ICC, including
-loneway- and -xtsum-, but it gives very different result. Are there any
restrictions for use of -xtsum-? I mean, can I use it for my data, which
is dichotomous data. As I wrote before, my data are unbalanced- clusters
contain between 44 and 111 patients. Here is the output:
. loneway diagnosis gpnum
One-way Analysis of Variance for diagnosis
Number of obs = 2647
R-squared = 0.0394
Source SS df MS
F Prob > F
------------------------------------------------------------------------
-
Between gpnum 11.486863 36 .31907954 2.97 0.0000
Within gpnum 280.36882 2610 .107421
------------------------------------------------------------------------
-
Total 291.85569 2646 .11030071
Intraclass Asy.
correlation S.E. [95% Conf. Interval]
------------------------------------------------
0.02684 0.00945 0.00833 0.04535
Estimated SD of gpnum effect .0544317
Estimated SD within gpnum .3277514
Est. reliability of a gpnum mean 0.66334
(evaluated at n=71.44)
. dis "ICC is " r(rho)
ICC is .02684102
. xtsum diagnosis, i(gpnum)
Variable | Mean Std. Dev. Min Max |
Observations
-----------------+----------------------------------------------------+-
---------------
diagnosis overall| 1.126181 .3321155 1 2 |
N = 2647
between | .0682964 1.025316 1.339286 |
n = 37
within | .3255142 .7868949 2.100864 |
T-bar = 71.5405
. dis "ICC is " r(sd_b)^2/(r(sd_b)^2+r(sd_w)^2)
ICC is .04216445
I hope you can give me some more details.
Greetings,
Ineta
Ineta Sokolowski <[email protected]> wrote:
> Can anybody explain, why the one-way ANOVA (-loneway-) and -xtsum-
gives
> different standard deviations for within and between effects of a
> variable?
>
> . loneway diagnosis gpnum
> . xtsum diagnosis, i(gpnum)
>
> where "diagnosis" is a dichotomous variable (0=no illness, 1=illness)
> and "gpnum" is the general practitioners (GP) number (38 different
> numbers). Each GP has different number of patients (between 44 and
111).
>
> How are the SD calculated in each procedure?
>
Fra: David M. Drukker, Stata Corp [mailto:[email protected]]
-xtsum- and -loneway- provide different summaries of the data. -xtsum-
is
summarizing the overall variable, the between transformed variable and
the
within transformed variable. The reported standard deviations are the
estimated standard deviations for the transformed variables. In
contrast,
-loneway- provides a one-way analysis of variance decomposition of the
specified variable. The formula for computing these standard deviations
are
standard in the ANOVA literature and documented in [R] loneway. It is
interesting to note that the reported standard deviations correspond to
the
variance components in a constant only model.
...
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