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

From   Nick Cox <>
To   "''" <>
Subject   RE: st: How to standardize a variable with a define mean
Date   Wed, 1 Feb 2012 11:23:18 +0000

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
> 25.8435
> . egen stdbmi=std(BMI), mean(meanbmi) std(1)
> option mean() incorrectly specified
> r(198);


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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