Re: st: RE: Taking averages, etc.

[Fred Wolfe <fwolfe@arthritis-research.org>]

> I'm probably just missing it, but it seems that Stata does not have a 
> convenient if-then-else structure, i.e. the equivalent of SPSS's DO IF 
> command.  In SPSS, I could do something like
>
> DO IF X1 = 1 and X2 = 3
>    Compute Y = 3.
> ELSE IF X3 = 2 and X4 = 17
>    Compute Y = 4.
> ELSE
>    Compute Y = 5.
> END IF.
>
> In SPSS, this would result in Y equaling 3, 4, 5, or missing for all 
> cases.  If, say, X1 = 1, X2 = 3, X3 = 2, X4 = 17, Y would be coded 3, i.e. 
> only the first compute would be executed.
>
> In stata, it seems to me like I would have to write increasingly 
> complicated if statements to achieve the same thing; something like
>
> gen y = 3 if X1==1 & X2==3
> replace y = 4 if X1 !=1 & X2 !=3 & X3==2 & X4==17
> replace y = 5 if X1 !=1 & X2 !=3 & X3 !=2 & X4 !=17
>
> or something like that.  Is that correct, or is there an easier way I am 
> overlooking?
I suspect the way most of us handle this type of problem (which we don't 
see as much of a problem at all) is to make new variables to define the if 
condition.

gen use1 = if X1 !=1 & X2 !=3 & X3==2 & X4==17
gen use2 = if X1 !=1 & X2 !=3 & X3 !=2 & X4 !=17

and so on. Then use these variables, as in replace y = 5 if use2

If the conditions are mutually exclusive, you can put the conditions into a 
single categorical variable.

Another way handle the problem is with a text editor where copying and 
replacing is simple.

Some of this


> DO IF X1 = 1 and X2 = 3
>    Compute Y = 3.
> ELSE IF X3 = 2 and X4 = 17
>    Compute Y = 4.
> ELSE
>    Compute Y = 5.
> END IF.
can be programmed into  a cond() function.

However, as Nick will probably tell you soon, there is much to said for 
simplicity and clarity. I prefer to define the if condition into individual 
variables together with textual notes or labels to state what the variables 
mean. Complicated code is all well and good when you are writing it, but 
when you come back to the program 6 months later the code is very difficult 
to understand.

Fred


Fred Wolfe
National Data Bank for Rheumatic Diseases
Wichita, Kansas
Tel (316) 263-2125     Fax (316) 263-0761
fwolfe@arthritis-research.org


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