Re: How to Calculate Interpolated Median (IM) Using Syntax

Posted by Bruce Weaver on
URL: http://spssx-discussion.165.s1.nabble.com/How-to-Calculate-Interpolated-Median-IM-Using-Syntax-tp5740421p5740425.html

I understood things differently from Art.  The following is quite rough and
ready, but I think it does what you describe.  (VECTOR and LOOP might be
tidier than DO-REPEAT, but I don't have a lot of time to mess around with it
any further right now.)  

HTH.


GET FILE='C:\Temp\SampleData.sav'.

* Get Nvalid and ordinary median for each question.
AGGREGATE
  /OUTFILE=* MODE=ADDVARIABLES
  /BREAK=
  /Nvalid1 Nvalid2 = NU(Q1 Q2)
  /med1 med2 = MEDIAN(Q1 Q2).

* n1 = number of scores less than Median (strictly less, not equal)
* n2 = number of scores equal to Median.

DO REPEAT n1 = n1Q1 n1Q2 / n2 = n2Q1 n2Q2 / med = med1 med2.
 COMPUTE n1 = Q1 LT med.
 COMPUTE n2 = Q1 EQ med.
END REPEAT.

* Sum those flag variables to convert them to counts.
AGGREGATE
  /OUTFILE=* MODE=ADDVARIABLES OVERWRITE=YES
  /BREAK=
  /n1Q1 n2Q1 n1Q2 n2Q2 = SUM(n1Q1 n2Q1 n1Q2 n2Q2).
FORMATS n1Q1 n2Q1 n1Q2 n2Q2 (F5.0).
DESCRIPTIVES n1Q1 n2Q1 n1Q2 n2Q2.

* Compute interpolated median (IM).

DO REPEAT
 IM = IM1 IM2 /
 med = med1 med2 /
 N = Nvalid1 Nvalid2 /
 n1 = n1Q1 n1Q2 /
 n2 = n2Q1 n2Q2.
DO IF n2 EQ 0.
 COMPUTE IM = med.
ELSE.
 COMPUTE IM = med-0.5+((0.5*N-n1)/n2).
END IF.
END REPEAT.
DESCRIPTIVES med1 med2 IM1 IM2.




asp778ACU-2 wrote

> Dear SPSS Base Gurus,
>
> I am trying to work something out which I am sure SPSS can do to calculate
> the IM
> N = total number of valid responses to the question
> M = the standard median of the scores
> N1 = number of scores less than M (strictly less, not equal)
> N2 = number of scores equal to M
>
> https://spu.edu/depts/insdev/interpolated_median_explanation.pdf
>  
> But I am not sure how to translate the IM formula above into the syntax.
> Attached dummy survey data set list that required interpolated median for
> Q1
> and Q2.
> What I try to achieve populate IM figures using syntax.
> As I am newbie with very basic knowledge of SPSS, any enlightenment will
> be
> appreciated.
>
> SampleData.sav
> <http://spssx-discussion.1045642.n5.nabble.com/file/t341643/SampleData.sav> 
> Thanks in advance.
>
>
>
>
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Bruce Weaver
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