Date: Thu, 27 Oct 2011 02:23:25 -0700
Reply-To: David Marso <david.marso@gmail.com>
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: David Marso <david.marso@gmail.com>
Subject: Re: Lag Function and Some Potentially Useless Syntax for a
Vertically Organized File
In-Reply-To: <CAB341xEXH8ZRMaDXC=rCbDq-2A6UdmBcbTY+H=AvSxxaJhk+Rw@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Step 1: Remove EXECUTE from your syntax!
Step 2: Examine your data.
Step 3: Don't use strings to represent numeric data.
Step 4: Get rid of unnecessary subscript on LAG (1 is default, hence not
required).
Step 5: DO IF is only required if multiple transformations are required for
a given conditional.
----
" However, I still don't
fully understand the meaning of T = (LAG(T,1) + 1)"
Because you have not done step 2. This is placing rather stringent
requirements on any success of "your" method. It is assuming that T is
followed by value T+1 in the file.
Revised: Assuming that T is NUMERIC as it *should* be and you already have
the sorted file active.
----
IF S EQ LAG(S) AND P EQ LAG(P) Xspt_LAG_1 = LAG(Xspt).
----
My 2 cents re PCP and syntax:
Point/Click/Paste is fine for generating templates, however general reliance
on such promises to result in a new generation of barely competent analysts!
---
WTF: How are you to document what the hell you did to achieve a given
result?
I could go on, but that would turn into a f'ing RANT!
--
Gregory Hildebrandt wrote:
>
> SPSS List Members::
>
> One issue that caused me difficult until I received help from a database
> programmer is how to lag a variable in a vertically organized file. I have
> provided some syntax below that can be used, which some may find helpful..
>
> For example, suppose the "Restructure Data" command may have been used to
> convert variables to cases, in order to permit the linear mixed model to
> be
> employed. This creates groups of cases, and it is not appropriate to
> directly use the lag command when computing the lagged value of the first
> member of a group. If this is done, the the last member of the prior
> group
> will be the computed lagged variable, and this is not correct. For the
> lagged value of the first member of a "group: "system missing" is the
> appropriate lagged value.
>
> For purposes of a slight generalization of the method, assume that time
> period, T, is a string variable. Restructure data will also properly sort
> the key hierarchical variables, but I will add the sort command for
> completeness.
>
> In this hierarchical, cross-section and time series vertically organized
> file, the variables School, S, Pupil, P, and the remaining variables are
> vertically organized. If X representing a particular variable in the file,
> which one desires to lag, each data point for this variable could be
> represented as Xspt, where s is a particular school, p is a particular
> student within school s, and t is the time period in which measurement
> occurs for a school, student combination. (Perhaps, notation could be
> sharper). The objective is to lag Xspt to obtain a series Xsp_l1 that
> returns a missing data point for the last member of the previous S, P, T
> combination.
>
> Here's the syntax:
>
> *step1.
> GET FILE='....sav'.
>
> *step2.
> Create a numeric T variable.
>
> COMPUTE T1=NUMBER(T,F4.0).
> execute.
> .
> *step 3.
> Sort the data by S, P, and T.
>
> *step 4 -- Compute Lag Xsp_l1.
> DO IF S=LAG(S,1) AND P=LAG(P,1) AND T=( LAG(T,1) + 1 ) .
> COMPUTE Xspt_l1 = LAG(Xspt).
> END IF.
> execute.
>
> Having used this syntax many times, I know it works. However, I still
> don't
> fully understand the meaning of T = (LAG(T,1) + 1), as it relates to this
> procedure, and interpretations would be appreciated.
>
> After several year of SPSS use, I've never gotten beyond point, click,
> paste
> and edit, and with the exception of a few specialized syntax commands,
> such
> as the above, this approach has met my requirements. Point, click and
> paste
> brings in the correct syntax, whereas writing syntax from scratch
> inevitably
> introduces a few typographical errors (even if one knows how to write the
> appropriate syntax) that can be hard to find. And, it is straightforward
> to
> identify how to edit the pasted syntax, if only changes in the model's
> variables are made.
>
> A number of years ago I took SPSS Tables from a world class SPSS Tables
> constructor/instructor who used syntax throughout the course to develop
> "Basic Tables." I was lost a good part of the time. However, when I took
> the course several years later, from the same instructor, who was then
> teaching "Custom Tables," even he was using point and click. He said he
> was
> not yet comfortable with the new syntax, and the course became quite easy
> to
> follow.
>
> Greg H
>
>
>
> On Wed, Oct 5, 2011 at 2:24 AM, John F Hall <johnfhall@> wrote:
>
>> Hereβs what it says on pp111,112 of the syntax reference guide****
>>
>> ** **
>>
>> ** **
>>
>> John F Hall****
>>
>> ** **
>>
>> johnfhall@ ****
>>
>> www.surveyresearch.weebly.com
>> <http://surveyresearch.weebly.com/>****
>>
>> ****
>>
>> ** **
>>
>> ** **
>>
>> ** **
>>
>> * *
>>
>> *LAG function*
>>
>> *LAG. *LAG(variable[, n]). Numeric or string. The value of variable in
>> the
>> previous case or n cases****
>>
>> before. The optional second argument, n, 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.****
>>
>> τ The result is of the same type (numeric or string) as the variable
>> specified as the first argument.****
>>
>> 112****
>>
>> *Universals*
>>
>> τ The first *n *cases for string variables are set to blanks. For
>> example, if PREV2=LAG****
>>
>> (LNAME,2) is specified, blanks will be assigned to the first two cases
>> for
>> *PREV2*.****
>>
>> τ When LAG is used with commands that select cases (for example, SELECT
>> IF and SAMPLE),****
>>
>> LAG counts cases *after *case selection, even if specified before these
>> commands. For more****
>>
>> information, see the topic Command Order on p. 41.****
>>
>> *Note*: In a series of transformation commands without any intervening
>> EXECUTE
>> commands or****
>>
>> other commands that read the data, lag functions are calculated after all
>> other transformations,****
>>
>> regardless of command order. For example,****
>>
>> COMPUTE lagvar=LAG(var1).****
>>
>> COMPUTE var1=var1*2.****
>>
>> and****
>>
>> COMPUTE lagvar=LAG(var1).****
>>
>> EXECUTE.****
>>
>> COMPUTE var1=var1*2.****
>>
>> yield very different results for the value of *lagvar*, since the former
>> uses the transformed value of****
>>
>> *var1 *while the latter uses the original value.****
>>
>> ** **
>>
>> *From:* SPSSX(r) Discussion [mailto:SPSSX-L@.UGA] *On Behalf
>> Of *joan casellas
>> *Sent:* 05 October 2011 10:13
>> *To:* SPSSX-L@.UGA
>> *Subject:* Lag Function****
>>
>> ** **
>>
>> Hi Everyone,****
>>
>> ** **
>>
>> Could someone explain to me what the LAG function does exactly? ****
>>
>> ** **
>>
>> * *
>>
>> * *
>>
>> *Joan *
>>
>> *Media Research Analyst*
>>
>> Phone: *+44 20 7593 1585* ****
>>
>> ** **
>>
>
--
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