I am going to measure the standarized mean value of 4 environmental indicators (AA, BB, CC and DD).

I want to take this value ((mean environmental value = (standard devi AA + sd BB + sd CC + sd DD)/4) for each country per each year.
For example mean environmental value for USA for year 2010.
Can somebody tell me how to use egen command for this purpose. Please see below my data (i have number of countries but here I included only few to explain my question)

Country Name environment indicator 2010 2011 2012
UK AA -1.32573 -1.34429 -1.26859
UK BB -1.1179 -1.15356 -0.98963
UK CC -0.22618 -0.36924 -0.38932
UK DD -1.03593 -1.09778 -0.96978
USA AA -1.26529 -1.26538 -1.26955
USA BB -1.12054 -1.12548 -1.07625
USA CC -0.66082 -0.59353 -0.85653
USA DD -0.59469 -0.5459 -0.51529
Australia AA 0.26126 0.318356 0.364368
Australia BB -0.33356 -0.34626 -0.3786
Australia CC -0.67317 -0.6633 -0.58567
Australia DD 0.314489 0.166391 0.124486
Burkina Faso AA -0.35061 -0.36786 -0.47889
Burkina Faso BB -0.56885 -0.55905 -0.62399
Burkina Faso CC -0.11764 -0.55538 -0.57313
Burkina Faso DD -0.1564 -0.17954 -0.11886
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