st: Carry forward an observation within a time frame

["Benigno Rodriguez, MD" <benignor@gmail.com>]

Dear David:

Thank you for this detailed response. But my problem is that I need 
to carry forward to a given row with a missing CD4 the closest in 
time CD4 value, as long as it is within the previous 4 months (the 
last 120 days is enough precision for my purposes).

Correct me if I'm thinking about this wrong, but with your approach, 
a value could be propagated down even if the "seed" for that value 
occurred more than 4 months earlier. I have added a few rows 
(artificially) to illustrate. Look at row 10. It occurs within 4 
month of row[_n-1], but I don't want it filled out with that value, 
because the last time we actually had a non-missing value was in row 
3, which is more than 4 months prior to row 10.

The approach I was trying was to try to define a "spell" as one that 
begins with a missing CD4 AND for which the row above has a 
non-missing value (easily done). But implementing the solution from 
there has proven challenging.


        patid   date            CD4     desired
1.      1007    5-May-55        .       .
2.      1007    1-Jan-00        .       .
3.      1007    3-Apr-02        5       5
4.      1007    8-Apr-02        .       5
5.      1007    11-Apr-02       .       5
6.      1007    13-May-02       .       5
7.      1007    30-May-02       .       5
8.      1007    30 Jun-02       .       5
9.      1007    30-Jul-02       .       5
10.     1007    30-Aug-02       .       .


BENIGNO RODRIGUEZ G., MD, MSc, FIDSA
Associate Professor of Medicine
Division of Infectious Diseases and HIV Medicine
Case Western Reserve University
Ph: +1-216-844-2057
Fax: +1-216-8445523  

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