Date:         Mon, 10 May 2004 13:17:32 -0500
Reply-To:     "Marks, Jim" <Jim.Marks@lodgenet.com>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         "Marks, Jim" <Jim.Marks@lodgenet.com>
Subject:      Re: selecting cases
Content-Type: text/plain; charset="us-ascii"

Jed:

In order to use LAG, you need the case with the event before the case
you want to flag in the data file-- i.e. you need to sort in descending
order of time/ interval.

The basic logic is:

1) sort by interval (and any other relevant grouping variables).

2) optional: create a "sortord" variable (I think it improves processing
time if there are less variables to sort on).

3) flag your cases of interest.

4) sort by Descending order of the sortord variable.

5) use LAG to the "previous" case with the event of interest.

6) resort to restore original order.

Here is sample syntax:

** sample data.
DATA LIST FREE /grp (F8.0) event (F8.0) date (ADATE10) time (TIME10).
BEGIN DATA
1 1 10/01/2001 09:18:03
1 2 10/01/2001 19:11:11
1 9 10/01/2001 15:15:15
2 2 10/01/2001 07:18:03
2 1 10/01/2001 19:11:11
2 9 10/11/2001 15:15:15
END DATA.

** sort and create sortord variable.
SORT CASES BY grp date time.
COMPUTE sortord EQ $CASENUM.

** flag cases of interest-- event = 9.
COMPUTE flgevent EQ event EQ 9.

** descending intervals.
SORT CASES by sortord (D).

** flag prior event.
COMPUTE prvevent EQ grp  EQ LAG(GRP) and LAG(event) eq 9.
RECODE prvevent (SYSMIS = 0).

** restore original order.
SORT CASES by sortord.

You can filter or select on the event of interest, and the prior event
using the variables flgevent and prevent.

cheers

--jim

-----Original Message-----
From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU]On Behalf Of
Jed J. Teres
Sent: Monday, May 10, 2004 12:42 PM
To: SPSSX-L@LISTSERV.UGA.EDU
Subject: selecting cases


Hello all.

I have an observation-level data set and I need to select observations
in
which a certain event occurs as well as the preceding interval.

Selecting the event intervals is easy- I can just say, "select if (event
eq
1)." How can I get the case immediately preceding this one?

Thanks,

Jed Teres