Re: st: How to standardize a variable with a define mean

[Alberto R Osella <arosella@irccsdebellis.it>]

Thanks NIck!!

Alberto R. Osella, MD, PhD
Laboratorio di Epidemiologia e Biostatistica
IRCCS Saverio de Bellis
Via Turi, 27
70013 Castellana Grotte (BA)
Italia
Tel:    +39 0804994655
Fax:    +39 0804994650
e-mail: arosella@irccsdebellis.it


Il 01/02/2012 12:23, Nick Cox ha scritto:
> Maarten is quite correct here in fixing your syntax, but note that -egen- is not needed here given that you have already -summarized- BMI.
>
> su BMI
> gen stdmi = (BMI - r(mean)) / r(sd)
>
> That way the whole business of putting value of r(mean) into a scalar, only to have to take it out again, is avoided.
>
> Even going the -egen- route the -scalar- is not needed and you don't even need to feed r(mean) to -egen- as by default the -std()- function works with the mean and standard deviation of the expression specified, here a variable name.
>
> . egen stdmpg1 = std(mpg)
>
> . su mpg
>
>      Variable |       Obs        Mean    Std. Dev.       Min        Max
> -------------+--------------------------------------------------------
>           mpg |        74     21.2973    5.785503         12         41
>
> . gen stdmpg2 = (mpg - r(mean)) / r(sd)
>
> . su stdmpg*
>
>      Variable |       Obs        Mean    Std. Dev.       Min        Max
> -------------+--------------------------------------------------------
>       stdmpg1 |        74   -7.00e-09           1  -1.606999    3.40553
>       stdmpg2 |        74   -7.00e-09           1  -1.606999    3.40553
>
> So, the problem needs one line or two, depending on whether you choose convenience or Stata minimalism, but not more.
>
> Nick
> n.j.cox@durham.ac.uk
>
> Maarten Buis
>
> On Wed, Feb 1, 2012 at 10:44 AM, Alberto R Osella wrote:
> > I want to standardize a variable (BMI) with a given mean value. I've
> > obtained that value from a -summarize- command, that is r(mean).
> > Now I want to pass the value of r(mean) to the egen command. Here the
> > commands and the error returned by Stata.
> >
> > . sum BMI
> > . scalar meanbmi=r(mean)
> > . display meanbmi
> > 25.8435
> > . egen stdbmi=std(BMI), mean(meanbmi) std(1)
> > option mean() incorrectly specified
> > r(198);
> try:
>
> egen stdbmi=std(BMI), mean(`=meanbmi') std(1)
>
> The trick is that meanbmi is the name of a scalar while the -mean()-
> option expects a number. By surrounding that scalar name with " `= "
> and " ' " you are forcing Stata to first evaluate the scalar name,
> resulting in a number, which is exactly what the -mean()- option
> wanted.
>
>
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