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st: RE: using creat count id on var


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: using creat count id on var
Date   Sun, 27 Feb 2005 19:50:36 -0000

This field centres on the use of -by:-. 

One attempt at an introductory overview 
emphasising the basics very strongly is 

How to move step by: step. 
Stata Journal 2(1): 86-102 (2002). 

This explains the use of the -by:- construct to tackle
a variety of problems with group structure, ranging from
simple calculations for each of several groups to more
advanced manipulations that use the built-in _n and _N. 

In addition, many data management and other FAQs 
contain examples of -by:-. 

In your case, you are over the last ridge and home 
is nearly in sight. As Joseph Coveney suggested, 
you also should tap into -egen- for handy functions. 

Again, there is propaganda in an overview 

On getting functions to do the work.
Stata Journal 2(4): 411--427 (2002). 

egen max1 = max(var1), by(id visitno)
egen mean1 = mean(var1), by(id visitno)

or 

bysort id visitno: egen max1 = max(var1) 
by id visitno: egen mean1 = mean(var1) 

Nick 
n.j.cox@durham.ac.uk 

Richard Hiscock
 
> Thanks Nick for your reply
> Im obviously finding it difficult to develop in this area
> I cant find a useful guide,any pointers?
> 
> My problem now is that having identified the visitno within 
> id I wish to 
> perform manipulations on a variable. For example
> using the illustrated structure.
> To obtain the maximum value of var1 within id at each level  
> of visitno.
> To obtain the mean value of var1 within id at each level of visitno
> 
>     id           date          visitno      var1
> >     1        16267         1             53
> >     1        16267         1            106
> >     1        16267         1            84
> >     1        16267         1            90
> >     1        16267         1            76
> >     1        16267         1            65
> >     1        16298         2             87
> >     1        16298         2            76
> >     2        15678         1            98
> >     2        15678         1            98
> >     2         16541        2            150

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