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RE: st: While/if for individual observations


From   "Williams, Barbara" <Barbara.Williams@vmmc.org>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   RE: st: While/if for individual observations
Date   Thu, 21 Mar 2013 16:49:01 +0000

Nick-
THANK YOU! For your reply to my question.
(I needed to think about my problem in a new way)
This is what I need ALMOST.

2 things:
1.   The code that you sent does not work-- invalid "l" (I have Stata 12):
. gen visitdate = date(VisitDate, "DMY")
. format visitdate %td
. tsset ID visitdate
       panel variable:  ID (weakly balanced)
        time variable:  visitdate, 01feb2011 to 10oct2011, but with gaps
                delta:  1 day
. capture ssc inst panelthin, replace
. panelthin, gen(start) min(28)
. bysort ID (visitdate) : gen spell = sum(start) l, sepby(ID)
invalid 'l' 
r(198);

2. For each spell, I want number of elapsed days since the "start":
generate elapsedays = 0 if start ==1
replace  elapsedays = X  if ???

Barb Williams

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
Sent: Thursday, March 21, 2013 5:07
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: While/if for individual observations

I can't follow all of this, but I have some suggestions which should help.

First off, a sufficient reason for your code to fail is that you cannot use subscripts in a -replace- statement, so that for example

replace abxin28days[`i'] = 1 + abxin28days[`i']

would need to be

replace abxin28days = 1 + abxin28days[`i'] in `i'

except that the subscript is not needed and

replace abxin28days = 1 + abxin28days in `i'

would do fine. (Code like this suggests knowing a lot about other languages, which does not always help.)

There is also some major confusion between local macros and variables, which are quite different beasts in Stata.

Second, there are cases in which a loop over observations is needed, and

SJ-7-3  pr0033  . . . . . . . . . . . . . .  Stata tip 51: Events in intervals
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
        Q3/07   SJ 7(3):440--443                                 (no commands)
        tip for counting or summarizing irregularly spaced
        events in intervals

Free .pdf at http://www.stata-journal.com/article.html?article=pr0033

discusses technique for problems a bit like yours. However, I am not clear that you actually need such an approach here (but the ideas may help in other problems).

Third, there are two kinds of manipulation bound together here, summarising antibiotic use and the idea that a lapse of 28 days implies a fresh start. I would rather take the second first, and here advertise -panelthin- (SSC) as addressing it.

Fourth, I don't get what your calculation is with the antibiotics, and don't see where numbers such as 4 and 6 come from. But using -egen- with -by()- options might be your answer. I have given you a -spell- variable.

Code so far

clear
input ID str9 VisitDate Antibiotics_YN
 100 01feb2011 0
 100 02feb2011 0
 100 04feb2011 1
 100 10may2011 1
 100 10oct2011 1
 101 01feb2011 0
 101 12mar2011 0
 101 13mar2011 0
 101 14mar2011 0
 101 18mar2011 1
 end
gen visitdate = date(VisitDate, "DMY")
format visitdate %td
tsset ID visitdate
capture ssc inst panelthin, replace
panelthin, gen(start) min(28)
bysort ID (visitdate) : gen spell = sum(start) l , sepby(ID)


     +--------------------------------------------------------+
     |  ID   VisitDate   Antibi~N   visitdate   start   spell |
     |--------------------------------------------------------|
  1. | 100   01feb2011          0   01feb2011       1       1 |
  2. | 100   02feb2011          0   02feb2011       0       1 |
  3. | 100   04feb2011          1   04feb2011       0       1 |
  4. | 100   10may2011          1   10may2011       1       2 |
  5. | 100   10oct2011          1   10oct2011       1       3 |
     |--------------------------------------------------------|
  6. | 101   01feb2011          0   01feb2011       1       1 |
  7. | 101   12mar2011          0   12mar2011       1       2 |
  8. | 101   13mar2011          0   13mar2011       0       2 |
  9. | 101   14mar2011          0   14mar2011       0       2 |
 10. | 101   18mar2011          1   18mar2011       0       2 |
     +--------------------------------------------------------+


On Wed, Mar 20, 2013 at 7:10 PM, Williams, Barbara <Barbara.Williams@vmmc.org> wrote:
> My data:
> I have a list of 20,000+ patient visits to a clinic with the following 3 variables:
>
> ID  VisitDate Antibiotics_YN
> 100 01feb2011 0
> 100 02feb2011 0
> 100 04feb2011 1
> 100 10may2011 1
> 100 10oct2011 1
> 101 01feb2011 0
> 101 12mar2011 0
> 101 13mar2011 0
> 101 14mar2011 0
> 101 18mar2011 1
> ...
>
> My question:
> I would like to create a new variable which labels Antibiotics WITHIN 28 days of their first visit.
> AND If the patient has another visit more than 28 days after the first visit, the time is re-set (new series).
> For ID=100 First series of visits beginning 01feb2011 Antibiotics on visitday = 4
>                     Second series of visits beginning 10may2011 Antibiotics on visitday = 1
>                    Third series of visits beginning 10oct2011 
> Antibiotics on visitday  =1 For ID=101 First series of visits beginning 01feb2011 No antibiotics
>                    Second series of visits beginning 12mar2011 
> Antibiotics on visitday = 6
>
>
> My syntax (doesn't run, I'm not sure how to do this):
> local i = 1
> local N = _N
> while `i' <=`N'   {
>    local firstdate = VisitDate[`i']
>    while ID[`i'] == ID[`i' - 1] & VisitDate[`i'] <= firstdate + 28 {
>        replace abxin28days[`i'] = 1 + abxin28days[`i']
>                    local `i' = `i' + 1 }
>    replace firstdate[`i'] = VisitDate[`i'] if _n==`i'
>    }
>
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