st: normalize variables

[Evangelos.Constantinou@warwick.ac.uk]

Hello,

I am using panel data analysis and I want to generate an index but first I
must normalise the variables (x1,x2) contained in the index. I normalised
them by the following set of commands:

egen min_x1=min(x1)
egen max_x1=max(x1)
gen normal_x1=(x1-min_x1)/(max_x1-min_x1)

So, my question is whether I need to transform the commands to include the
"by(.)" option i.e.

egen min_x1=min(x1), by(.)
egen max_x1=max(x1), by(.)
gen normal_x1=(x1-min_x1)/(max_x1-min_x1)

and if so, should i include the panel or time variable.

Thanks
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