using lag function to keep records based on certain conditions

[thara vardhan-2]

Dear List Members

Can anyone teach me how to use the lag function to select records in my
data file based on certain conditions please?

Situation1:

Erefnum   Personcni     Newifc  newweapon

1234          8888                 5                21
1234          8888                 5                21

I need to keep record 1 only in my data file.

Situation2:

Erefnum   Personcni     Newifc  newweapon

5678          9999                 1                7
5678          9999                 1                11

In this case I need to keep the record with newweapon code 11 in my data
file.

I am using SPSS version17.0 since last week. Using the menu I tried to use
transform/shift values function but was not able to do it.

Any help is much appreciated.

thanks
regards
Thara Vardhan
Senior Statistician
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[Simon Palmer-4]

Hi Thara,

There may be some clever new way of doing this, but these have worked
since before SPSS moved to windows:

> Situation1:
>
> Erefnum   Personcni     Newifc  newweapon
>
> 1234          8888                 5                21
> 1234          8888                 5                21
>
> I need to keep record 1 only in my data file.

I'm assuming you're identifying this by the erefnum, and that you want
to keep the first record with each erefnum only.

select if erefnum ne lag(erefnum).
execute.

If your keep criterion is more complex you might need to do something
like:

select if erefnum = lag(erefnum) and personcni = lag(personcni) and
newifc = lag(newifc) and newweapon = lag(newweapon).
execute.

An alternative is to use aggregate:

agg outfile * /break erefnum /personcni = first(personcni) /newifc =
first(newifc) / newweapon= first(newweapon).
execute.

>
>
> Situation2:
>
> Erefnum   Personcni     Newifc  newweapon
>
> 5678          9999                 1                7
> 5678          9999                 1                11
>
> In this case I need to keep the record with newweapon code 11 in my
> data
> file.

Assuming you want to keep the last record for a particular erefnum:

sort cases erefnum.
compute recnum = $casenum.
compute keepnext = 0.
if erefnum ne lag(erefnum) keepnext = 1.
sort cases recnum (d).
select if lag(keepnext) = 1.
sort cases recnum (a).

In this case aggregate would be much easier:

agg outfile * /break erefnum /personcni = last(personcni) /newifc =
last(newifc) / newweapon= last(newweapon).
execute.

Cheers,
Simon

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[Richard Ristow]

At 09:26 PM 11/30/2008, Thara Vardhan wrote:

>Can anyone teach me how to use the lag function
>to select records in my data file based on certain conditions please?

Did you ever get a solution to this? I'm not
attempting one; I don't understand just what your
selection logic is meant to be.

Selecting records using LAG is tricky, and
sometimes requires using EXECUTE. See example
below, and section "Use EXECUTE Sparingly" in any
edition of "SPSS® Programming and Data Management".

|-----------------------------|---------------------------|
|Output Created               |06-DEC-2008 15:48:22       |
|-----------------------------|---------------------------|
Group Letter

    1  A
    1  B
    1  C
    2  D
    3  E
    3  F
    3  G
    3  H
    4  I

Number of cases read:  9    Number of cases listed:  9


SELECT IF Group NE LAG(Group).
LIST.
List
|-----------------------------|---------------------------|
|Output Created               |06-DEC-2008 15:48:58       |
|-----------------------------|---------------------------|

Number of cases read:  0    Number of cases listed:  0

=============================
APPENDIX: Test data, and code
=============================
DATA LIST FIXED /
    Group  03-04
    Letter 06    (A).
*---|---10----|---20.
BEGIN DATA
    1 A
    1 B
    1 C
    2 D
    3 E
    3 F
    3 G
    3 H
    4 I
END DATA.

LIST.

SELECT IF Group NE LAG(Group).
LIST.

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