st: Problem with stset and sttocc

[Andrew Hall <A.Hall04@westminster.ac.uk>]

Hello

I am having problems using stset and sttocc to create a case-control 
data set from a simple cross-sectional data set.  Sttocc is selecting 
cases as controls.   I am using Intercooled STATA 9.2 on Windows 
XP.  This is what is happening.

I have cross-sectional survey data on 7,572 Ethiopian schoolchildren 
of whom 1,283 are orphans and 6,289 are non-orphans.  The variable 
orphan is coded as 1 = orphan, 0 = non orphan.

The data were collected between 28/11/2006 and 8/2/2007.

I want to randomly select a non-orphan for each orphan matched on sex 
and age at least to create a case-control data set.

I have stset the dataset using either date of visit (dov1) as the 
time variable or created a new time variable fixed on one arbitrary 
date e.g. 01/01/2007 (dovfixed)

               stset  dov1, failure(orphan=1)
  or          stset dovfixed, failure(orphan=1)

This creates new temporary variables including _d which cross-tabs 
perfectly with orphan (_d = 1 and orphan = 1 n=1283, _d =0 and orphan 
= 0, n=6289).  It seems that the dataset have been properly stset (or has it?).

I then use sttocc to match each case to one control on the variables 
sex (1=male; 2=female) and ageyrs (in years) as follows:

            sttocc, match (sex ageyrs) number(1)

This works and cannot find controls for 2 cases only.

But when I do a cross-tab of orphan  by _case I find that 278 
controls who should be non-orphans have been selected from the cases 
(orphans, failure=1).  All controls should be selected from the non-orphans.

Snapspan has no effect on the dataset as all id numbers in the data 
set are unique anyway.

Why are orphans (failure) being selected as controls for orphans 
(failure) when I have specified non-orphans? Am I just being dim?  Is 
there something wrong the way I'm using stset?  I have tried setting 
origin, enter and exit, but they are not really relevant as all 
subjects were in effect studied on the same day, so it is not 
time  series data.  I am something of a novice and can't find any 
similar issues discussed on the listserv archives, hence my request..

Thanks for reading this and suggestions would be gratefully received.


Andrew Hall MSc PhD RPHNutr

Reader in Public Health Nutrition
Centre for Public Health Nutrition
Westminster University
115 New Cavendish Street
London W1W 6UW

Tel: + 44 (0)207 911 5000 Ext 3910  

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