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
[email protected]
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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
fax 61 8 8223 4075
http://www.public-health.adelaide.edu.au/
CRICOS Provider Number 00123M
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