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From |
Rachele Capocchi <rachele.capocchi@arsanita.toscana.it> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: CI for adjusted mean |

Date |
Thu, 25 Oct 2007 10:57:45 +0200 |

Hi Ken,

sorry but maybe I don't understand what you explain in your mail.

I tried tried:

. adjust wei, by(rep) se ci

and I obtained the adjusted means of the groups, but I don't know how I can obtain the overall mean.

Is there a command to do this ?

Thanks a lot,

Rachele

Rachele Capocchi

Agenzia Regionale Sanità Toscana

"U.O. Centro Statistico Elaborazione Dati"

Sede Operativa - viale Milton,7

50134 Firenze

Tel. 055-4624354

Fax. 055-4624345

khigbee@stata.com ha scritto:

Maarten buis <maartenbuis@yahoo.co.uk> asks:

... I expected that this code:The description paragraph for -adjust- (manual or help file) says:

sysuse auto, clear

recode rep78 1/2 = 3

anova mpg rep78 wei, continuous(wei)

adjust wei, se ci

would give me a mean of mpg adjusted for wei (with standard error and

confidence interval), but instead it gives me an empty table. Does

anyone have any thoughts on this one?

... If by() is not specified, adjust produces results as if

by() defined one group. Variables used in the estimation

command but not included in either the by() variable list or

the adjust variable list are left at their current values,

observation by observation. Here adjust displays the average

estimated prediction (or the corresponding probability or

exponentiated prediction), substituting the mean of these

unspecified variables within each group defined by the

variables in the by() option.

Since -by()- was not specified, we are dealing with one group

(the whole estimation sample) and since rep78 was not specified

in the -adjust- command, the mean of rep78 over the whole

estimation sample is used in the prediction (see the last part of

the description "... substituting the mean ...").

With your example the mean of rep78 over the estimation sample is

3.57971. After an -anova- where rep78 is in the model as a

categorical variable, -predict- (and hence -adjust- which uses

-predict-) returns a missing value when asked for the prediction

of the model where the categorical variable is set to a value

that was not one of the levels of that categorical variable.

That is why you end up with an empty table--the -predict- command

returned a missing value for the prediction.

You could instead do things like

. adjust wei rep=3, se ci

. adjust wei rep=4, se ci

. adjust wei rep=5, se ci

or more easily

. adjust wei, by(rep) se ci

With these cases, the rep78 variable is treated as a categorical

variable (like it should be in this -anova- example).

Ken Higbee khigbee@stata.com

StataCorp 1-800-STATAPC

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**Follow-Ups**:**Re: st: CI for adjusted mean***From:*Maarten buis <maartenbuis@yahoo.co.uk>

**References**:**Re: Re: st: CI for adjusted mean***From:*khigbee@stata.com

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