Re: logical operations on dates/times

[Kylie]

Sorry, I probably should have said how the date variables were created.

Time was imported from Excel where it was in h:mm format. V4_48_B was
computed in SPSS using the TIME.HMS function on two separate numeric
variables also imported from Excel. Ie, COMPUTE
V4_48_B=TIME.HMS(V4_48_B_hr, V4_24_48_min, 0).

Thanks,
Kylie.


-----Original Message-----
From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of
Kylie Lange
Sent: Tuesday, 15 December 2009 10:26 AM
To: [hidden email]
Subject: logical operations on dates/times

Hi everyone,

I am currently working on a file where I have to compare date variables and
flag those cases that are a match. I am running into problems due to the
internal precision of the variables, which means that cases that I want to
be considered equal, are not. Here is what I have:

Name Visit Period24 Time V4_48_B
HC    d     2       9:25   9:40
HC    d     2       9:30   9:40
HC    d     2       9:35   9:40
HC    d     2       9:40   9:40
HC    d     2       9:45   9:40
TD    d     2       7:58   8:10
TD    d     2       8:03   8:10
TD    d     2       8:08   8:10
TD    d     2       8:13   8:10
TD    d     2       8:18   8:10

V4_48_B holds the time that the subject had breakfast at during that period
of that visit. I want to create a variable that flags the row where Time
occurs at, or the first one after, breakfast (V4_48_B and Time are both Date
type variables). Ie, I want:

Name Visit Period24 Time V4_48_B V4_48_B_flag
HC    d     2       9:25   9:40     0
HC    d     2       9:30   9:40     0
HC    d     2       9:35   9:40     0
HC    d     2       9:40   9:40     1
HC    d     2       9:45   9:40     0
TD    d     2       7:58   8:10     0
TD    d     2       8:03   8:10     0
TD    d     2       8:08   8:10     0
TD    d     2       8:13   8:10     1
TD    d     2       8:18   8:10     0

Here is the syntax I have been using:

COMPUTE V4_48_B_flag=0.
IF (Name=LAG(Name) & Visit=LAG(Visit) & Visit='d' & Period24=2 & (V4_48_B GT
LAG(Time)) &  (V4_48_B LE Time)) V4_48_B_flag=1.

However, the fourth row is not being flagged as a match because there is a
small precision difference between Time and V4_48_B. Computing the
difference of these two variables shows that the difference for the fourth
row is -7.276e-12. Thus the flag variable isn't being set to 1 until it
reaches the following row, with Time=9:45, as the final LE clause in the IF
statement isn't true for the Time=9:40 row.

Can anyone suggest what I need to do to get the behaviour that I want? Can I
round/truncate the Time or V4_48_B variables? Or change the IF statement in
some way? (This is of course just a subset of the total data file, which has
data from 4 visits of 3 periods each for each subject, each period being 24
hours long, and I am doing this matching/flagging process for three meals in
each 24 hour period.)

Thanks for any suggestions,
Kylie.

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