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Re: st: SUM question


From   Svend Juul <SJ@SOCI.AU.DK>
To   <statalist@hsphsun2.harvard.edu>
Subject   Re: st: SUM question
Date   Thu, 15 May 2008 09:00:05 +0200

Satomi wrote:

...
I think  egen newvar = rowtotal(var1 var2) is equivalent to replacing
missing values with 0(zero).

The SUM function in SPSS adds cases in the variable where at least one
observation is valid:

Var 1   Var2
1        4
3        5
4        5
5        8
.        5
.        .

When  compute newvar= sum(var1, var2). is used, there will be 1 missing
case.
When compute newvar= var1+var2. is used, there are 2 missing cases.

=================================================================

You can gain control with -egen-'s -rowmiss()- and -rownonmiss()- 
functions:

   . generate plusvar = var1+var2
   . egen egenvar1 = rowtotal(var1 var2)
   . egen nomiss = rownonmiss(var1 var2)
   . generate egenvar2 = egenvar1 if nomiss>0

   . list , clean

          var1   var2   plusvar   egenvar1   nomiss   egenvar2  
     1.      1      4         5          5        2          5  
     2.      3      5         8          8        2          8  
     3.      4      5         9          9        2          9  
     4.      5      8        13         13        2         13  
     5.      .      5         .          5        1          5  
     6.      .      .         .          0        0          .  

Hope this helps
Svend
__________________________________________

Svend Juul
Institut for Folkesundhed, Afdeling for Epidemiologi
(Institute of Public Health, Department of Epidemiology)
Vennelyst Boulevard 6
DK-8000  Aarhus C, Denmark
Phone:  +45 8942 6090
Home:   +45 8693 7796
Email:  sj@soci.au.dk
__________________________________________ 

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