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

From   Alberto R Osella <>
Subject   Re: st: How to standardize a variable with a define mean
Date   Wed, 01 Feb 2012 12:34:11 +0100

Thanks NIck!!

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

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.


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
. egen stdbmi=std(BMI), mean(meanbmi) std(1)
option mean() incorrectly specified

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

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