Re: st: RE: Duplicate combinations of variables

[Miranda Kim <mk@mrc.soton.ac.uk>]

Thanks a lot Nick!

Nick Cox wrote:
> Assuming that your identifiers are numeric, as implied here, then 
>
> gen minID = min(CaseID, ControlID)
> gen maxID = max(CaseID, ControlID) 
> duplicates report minID maxID 
>
> and this check will do no harm: 
>
> assert CaseID != ControlID 
>
> For more on the min-max trick here, see e.g. 
>
> SJ-8-4  dm0043  . Tip 71: The problem of split identity, or how to group
> dyads
>         . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N.
> J. Cox
>         Q4/08   SJ 8(4):588--591                                 (no
> commands)
>         tip on how to handle dyadic identifiers
>
> Nick 
> n.j.cox@durham.ac.uk 
>
> Miranda Kim
>
> I am a Stata 11 user, and I wonder if anyone has advice on achieving the
>
> following:
> I have two variables, one with ID numbers for cases and one with ID 
> numbers for matched controls in a case control study. There are 3 
> controls matched to each case, and a subject may serve as a control for 
> more than one case or later become a case. I am wanting to check that in
> my dataset all the pairs of cases and their matched control are unique; 
> ie that I don't have for example:
> Case ID Control ID
> 123 253
> 253 123
>
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>
>   
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