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Re: handling date data (was st: Please help!)
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
[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
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.
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.
Neurosciences Institute, Austin Health
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