Date:         Mon, 16 Jan 2006 15:37:40 +0100
Reply-To:     Spousta Jan <JSpousta@CSAS.CZ>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Spousta Jan <JSpousta@CSAS.CZ>
Subject:      Re: Percentiles
Content-Type: text/plain; charset="us-ascii"

Yes, thank you - I wrote already to Art Kendall who devised this idea an
hour ago that in the morning after the busy weekend I am usually not
able to remember all the important things I should keep in mind :-)

Greetings

Jan

-----Original Message-----
From: Peck, Jon [mailto:peck@spss.com]
Sent: Monday, January 16, 2006 3:31 PM
To: Spousta Jan; SPSSX-L@LISTSERV.UGA.EDU
Subject: RE: Re: [SPSSX-L] Percentiles

Remember that the RANK command can compute the ranks and percentiles as
a new variable, after which you can these values in transformations.

Regards,
Jon Peck
SPSS

-----Original Message-----
From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On Behalf Of
Spousta Jan
Sent: Monday, January 16, 2006 3:02 AM
To: SPSSX-L@LISTSERV.UGA.EDU
Subject: Re: [SPSSX-L] Percentiles

Hi Luca and all,

I think that Greg's solution is very nice and simple, but has severe
limitations: It is valid only if the variable x is Gaussian and the
number of cases is huge - then the asymptotics works.

But in general case one must apply exact algorithms, i.e. compute the
true values of percentiles first. Here is an example:

* save the percentile in a file.
GET FILE='C:\Program Files\SPSS13\GSS93 subset.sav'.
oms /SELECT TABLES /IF LABELS = ["Statistics"]
  /DESTINATION FORMAT=sav OUTFILE = 'C:\Program
Files\SPSS13\auxfile.sav'.
FREQUENCIES age /FORM=NOTABLE /PERC= 20.
omsend.
get file='C:\Program Files\SPSS13\auxfile.sav'.
select if Var1 = "Percentiles".
save outfile='C:\Program Files\SPSS13\auxfile.sav'.

* open the original file again and merge the percentile.
GET FILE='C:\Program Files\SPSS13\GSS93 subset.sav'.
string Var1 (a11).
compute Var1 = "Percentiles".
MATCH FILES /FILE=*
 /TABLE='C:\Program Files\SPSS13\auxfile.sav'
 /BY Var1 /drop Command_ Subtype_ Label_ Var2 Var3.
EXECUTE.

* compute the transformed variable y.
compute y = age <= Var4.
exe.
var lab y "Age is <= 20th percentile of Age (1-0)".
form y (f2).
del var Var1 Var4.

Greetings

Jan

-----Original Message-----
From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On Behalf Of
Meyer, Gregory J
Sent: Saturday, January 14, 2006 6:51 PM
To: SPSSX-L@LISTSERV.UGA.EDU
Subject: Re: Percentiles

Luca, assuming you are interested in using a sample determined
percentile, here's one way to do what you want.

* Convert observed scores to standardized Z scores
  using the M and SD from your sample .

DESCRIPTIVES
  VARIABLES= X (X_zscore) /SAVE .

* Because a Z score of -.84 cuts off the lowest 20% of the
  distribution, recode based on the X_zscore variable.

RECODE
  r_zscore
  (Lowest thru -.84 = 1)  (ELSE = 0)  INTO  Y .
EXECUTE .

* Note, the cut off value can be modified to any percentile you desire
  using the normal curve equivalents found in the back of any stats book
.

Greg

| -----Original Message-----
| From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On Behalf
| Of Luca Meyer
| Sent: Saturday, January 14, 2006 2:59 AM
| To: SPSSX-L@LISTSERV.UGA.EDU
| Subject: Percentiles
|
| Is there any easy way to compute a X percentile of a variable? What i
| need is something like:
|
| IF X<=PERCENTILE(20) Y=1.
|
| That is: if the value of X for each single case is lower or equal to
| the 20% percentile of X then Y=1.
|
| Is this somehow possible?
|
| Thanks,
| Luca
|
| Mr. Luca MEYER
| Survey research, data analysis & more: http://www.lucameyer.com/
| Tel: +390122854456 - Fax: +391782232575 - Mobile: + 393394950021
|