SPSSX Discussion

lag function

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Alina Sheyman-3

lag function

Is it possible to make lag function more complex? For instance if the second
observation if i am looking for is the first observation +1. I've tried the
following syntax and it doesn't seem to work

IF start_date=LAG(start_date+1)) STCNT=LAG(STCNT) +1.

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Melissa Ives

Re: lag function

Yes. This is from the SPSS help.
LAG(arg,n) The value of the variable n cases before. The first argument is a variable. The second argument, if specified, is a constant and must be a positive integer; the default is 1. For example, PREV4=LAG(GNP,4) returns the value of GNP for the fourth case before the current one. The first four cases have system-missing values for PREV4.

So to go back to the 2nd lag you would do lag(arg,2).

Melissa

-----Original Message-----
From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of Alina Sheyman
Sent: Wednesday, October 08, 2008 11:12 AM
To: [hidden email]
Subject: [SPSSX-L] lag function

Is it possible to make lag function more complex? For instance if the second observation if i am looking for is the first observation +1. I've tried the following syntax and it doesn't seem to work

IF start_date=LAG(start_date+1)) STCNT=LAG(STCNT) +1.

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Alina Sheyman-3

Re: lag function

The problem is that I don't want the value of the second case preceding my
observation, but rather the value of the first case preceding the
observation - 1

IF start_date=LAG(start_date-1)) STCNT=LAG(STCNT) +1.
ie. st_date stcnt
12-1 1
10-2 2
7-3 3
so in this example if the next start date is 7-2, a new stcnt value of 4
should be returned, if not then not

the new observation should be (10-1)

On Wed, Oct 8, 2008 at 12:15 PM, Melissa Ives <[hidden email]> wrote:

> Yes. This is from the SPSS help.
> LAG(arg,n) The value of the variable n cases before. The first argument is
> a variable. The second argument, if specified, is a constant and must be a
> positive integer; the default is 1. For example, PREV4=LAG(GNP,4) returns
> the value of GNP for the fourth case before the current one. The first four
> cases have system-missing values for PREV4.
>
> So to go back to the 2nd lag you would do lag(arg,2).
>
> Melissa
>
> -----Original Message-----
> From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of
> Alina Sheyman
> Sent: Wednesday, October 08, 2008 11:12 AM
> To: [hidden email]
> Subject: [SPSSX-L] lag function
>
> Is it possible to make lag function more complex? For instance if the
> second observation if i am looking for is the first observation +1. I've
> tried the following syntax and it doesn't seem to work
>
> IF start_date=LAG(start_date+1)) STCNT=LAG(STCNT) +1.
>
> =====================
> To manage your subscription to SPSSX-L, send a message to
> [hidden email] (not to SPSSX-L), with no body text except the
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> commands to manage subscriptions, send the command INFO REFCARD
>
> PRIVILEGED AND CONFIDENTIAL INFORMATION
> This transmittal and any attachments may contain PRIVILEGED AND
> CONFIDENTIAL information and is intended only for the use of the
> addressee. If you are not the designated recipient, or an employee
> or agent authorized to deliver such transmittals to the designated
> recipient, you are hereby notified that any dissemination,
> copying or publication of this transmittal is strictly prohibited. If
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>

Spousta Jan

Re: lag function

Perhaps I misunderstand the question, but what about something like this:

IF (start_date=LAG(start_date)+1) STCNT=LAG(STCNT) +1.

(Of couse if "date" is really a date, you should use date arithmetic - functions like datediff.)

Jan

-----Original Message-----
From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of Alina Sheyman
Sent: Wednesday, October 08, 2008 6:25 PM
To: [hidden email]
Subject: Re: lag function

The problem is that I don't want the value of the second case preceding my observation, but rather the value of the first case preceding the observation - 1

IF start_date=LAG(start_date-1)) STCNT=LAG(STCNT) +1.
ie. st_date stcnt
12-1 1
10-2 2
7-3 3
so in this example if the next start date is 7-2, a new stcnt value of 4 should be returned, if not then not

the new observation should be (10-1)

On Wed, Oct 8, 2008 at 12:15 PM, Melissa Ives <[hidden email]> wrote:

> Yes. This is from the SPSS help.
> LAG(arg,n) The value of the variable n cases before. The first
> argument is a variable. The second argument, if specified, is a
> constant and must be a positive integer; the default is 1. For
> example, PREV4=LAG(GNP,4) returns the value of GNP for the fourth case
> before the current one. The first four cases have system-missing values for PREV4.
>
> So to go back to the 2nd lag you would do lag(arg,2).
>
> Melissa
>
> -----Original Message-----
> From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf
> Of Alina Sheyman
> Sent: Wednesday, October 08, 2008 11:12 AM
> To: [hidden email]
> Subject: [SPSSX-L] lag function
>
> Is it possible to make lag function more complex? For instance if the
> second observation if i am looking for is the first observation +1.
> I've tried the following syntax and it doesn't seem to work
>
> IF start_date=LAG(start_date+1)) STCNT=LAG(STCNT) +1.
>
> =====================
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> list of commands to manage subscriptions, send the command INFO
> REFCARD
>
> PRIVILEGED AND CONFIDENTIAL INFORMATION This transmittal and any
> attachments may contain PRIVILEGED AND CONFIDENTIAL information and is
> intended only for the use of the addressee. If you are not the
> designated recipient, or an employee or agent authorized to deliver
> such transmittals to the designated recipient, you are hereby notified
> that any dissemination, copying or publication of this transmittal is
> strictly prohibited. If you have received this transmittal in error,
> please notify us immediately by replying to the sender and delete this
> copy from your system. You may also call us at (309) 827-6026 for
> assistance.
>

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Richard Ristow

Re: lag function

In reply to this post by Alina Sheyman-3

At 12:12 PM 10/8/2008, Alina Sheyman wrote:

>Is it possible to make lag function more complex? For instance if
>the second observation if i am looking for is the first observation
>+1. I've tried the following syntax and it doesn't seem to work
>
>IF start_date=LAG(start_date+1)) STCNT=LAG(STCNT) +1.

To start with, you have a missing left parenthesis; I suppose you mean

IF (start_date=LAG(start_date+1)) STCNT=LAG(STCNT) +1.

Melissa's answer may have given you what you need; but I'm not sure,
since I can't tell what you are looking for.

"STCNT=LAG(STCNT) +1" is a valid computation, either in a COMPUTE or
an IF statement. Be careful, though, if you execute it conditionally,
in an IF or a DO IF: In some logic, when it's not executed, STCNT is
left system-missing, and all later values of STCNT are missing as well.

"LAG(start_date+1)" is >not< a valid use of the LAG function; the
argument (or first argument) to LAG must be a variable, not an
expression. This is where I lose your meaning: what did you want,
using LAG that way?

-Good luck,
Richard

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Alina Sheyman-3

Re: lag function

this is the statement that I'm trying to use

IF (start_date=(LAG(range(datediff(start_date, ,"days"),1,3))
STCNT=LAG(STCNT) +1).

ie. st_date stcnt
12-1 1
10-2 2
7-3 3

so in this example if the next start date is in the range of 1-3 days from
7-3, a new stcnt value of 4 should be returned, if not then not

On Wed, Oct 8, 2008 at 12:41 PM, Richard Ristow <[hidden email]>wrote:

> At 12:12 PM 10/8/2008, Alina Sheyman wrote:
>
> Is it possible to make lag function more complex? For instance if the
>> second observation if i am looking for is the first observation +1. I've
>> tried the following syntax and it doesn't seem to work
>>
>> IF start_date=LAG(start_date+1)) STCNT=LAG(STCNT) +1.
>>
>
> To start with, you have a missing left parenthesis; I suppose you mean
>
> IF (start_date=LAG(start_date+1)) STCNT=LAG(STCNT) +1.
>
>
> Melissa's answer may have given you what you need; but I'm not sure, since
> I can't tell what you are looking for.
>
> "STCNT=LAG(STCNT) +1" is a valid computation, either in a COMPUTE or an IF
> statement. Be careful, though, if you execute it conditionally, in an IF or
> a DO IF: In some logic, when it's not executed, STCNT is left
> system-missing, and all later values of STCNT are missing as well.
>
> "LAG(start_date+1)" is >not< a valid use of the LAG function; the argument
> (or first argument) to LAG must be a variable, not an expression. This is
> where I lose your meaning: what did you want, using LAG that way?
>
> -Good luck,
> Richard
>
>

Richard Ristow

Re: lag function

At 12:49 PM 10/8/2008, Alina Sheyman wrote:

>this is the statement that I'm trying to use
>
>IF (start_date=(LAG(range(datediff(start_date, ,"days"),1,3))
> STCNT=LAG(STCNT) +1).

You're confused several ways, here.

First, as you've found, you can only lag a variable, not a computed
quantity like "range(datediff(start_date, ,"days"),1,3)"

Second, that isn't a valid computed quantity, anyway. "datediff"
takes three arguments, and in your expression, you don't have the second one.

You probably mean something like,

IF RANGE(DATEDIFF(start_date,LAG(start_date),0,3)
STCNT=LAG(STCNT) +1).

But that won't work for you either, since in the data below,
>ie. st_date stcnt
> 12-1 1
> 10-2 2
> 7-3 3

'st_date' is not an SPSS date value, and DATEDIFF only works with
SPSS date values.

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