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Re: handling date data (was st: Please help!)


From   Lisa Sharwood <lisans@unimelb.edu.au>
To   statalist@hsphsun2.harvard.edu
Subject   Re: handling date data (was st: Please help!)
Date   Mon, 17 May 2004 15:56:51 +1000

Hello Lisa

Welcome to Stata.

(Just an initial tip: a meaningful Subject: header is likely to lead to a faster response from the list than a generic "Please help!".)

You have, I surmise, 2 variables, call them timeA and timeB, which represent each observation's date of entry to the study and exit from the study respectively. Given the dates are in Stata's elapsed form:

generate studytime = timeB - timeA

gives you a new variable, studytime, that represents the time-on-study for _every_ observation (ie each subject). Stata does not need to explicitly loop through observations to do this type of thing - it does the operation on every observation unless you instruct otherwise. You instruct otherwise using the -if- or -in- command qualifiers (vide infra). studytime will, by default, be a floating point variable and you may wish, since time in days is integer, choose another data type, for example:

generate long studytime = timeB - timeA

to give studytime a "long integer" type. see help datatypes; maybe type "int" will be sufficient for your needs?

You can then use Stata's -list- -summarize- and other commands to look at the studytime.

[Actually, you now have a time-on-study variable, studytime, so you could jump straight into the world of survival analysis. All you need to -stset- your data, the passport into that world, is such a time variable. See -help st- and thence -help stset-.]



If you wished to generate the studytime for only subjects 108 through 215 then you could:

generate long studytime = timeB - timeA in 108/215

but this is very dependent on the current sort order of the data so be sure that these really are the subjects on whom you wish to calculate the new variable. All other observations will be accorded a missing value on studytime.
It is probably safer to give these subjects a flag or indicator based on the substantive reason why they are important as a subgroup and use that indicator to select on subsequent manipulations and analyses.

I hope this helps. New users of Stata often benefit greatly by taking one of the Stata "Netcourses". I note from http://www.stata.com/news/nc.html that an introductory course begins in June. Might be worth a look.

Phil


At 04:38 PM 11/05/2004 +1000, you wrote:

Hello and thank you for attending to my enquiry!!
I am a very new Stata user and plowing through the user guide and reference manuals to try and perform a calculation.... and I need help please!

I have two variables for which I have converted the list of dates into real elapsed dates. I want to subtract one from the other and thus generate a new variable with that figure.. which will give me the amount of time the person has stayed in the study. I have managed to
'compare' the two variables but only in one observation at a time. Could you please tell me how to do it for all the observations, and can I use 'in 108/215' for example.
I hope this is understandable and I hope to hear from you soon. My ultimate aim is to perform Kaplan Mieir survival analysis on the data.

Thank you

Lisa Sharwood
Research Officer
Neurosciences Institute, Austin Health
Melbourne University
lisans@unimelb.edu.au
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Philip Ryan
Associate Professor,
Department of Public Health
Associate Dean (Information Technology)
Faculty of Health Sciences
University of Adelaide 5005
South Australia
tel 61 8 8303 3570
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http://www.public-health.adelaide.edu.au/
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