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st: -alpha-, unexpected behavior


From   Joerg Luedicke <joerg.luedicke@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: -alpha-, unexpected behavior
Date   Thu, 22 Mar 2012 21:10:02 -0700

Hi everyone!

I am puzzled by something related to Stata's -alpha-, a program
which, for example, calculates Cronbach's alpha or can be used to
create a variable that holds the arithmetic mean of some other variables.
I am puzzled because when wrapping -alpha- into some program, it
does not behave as expected.

//To illustrate the problem, let's first get some toy data:
use "http://www.stata-press.com/data/r11/bg2";, clear

//and create 100 missing values:
replace bg2cost1=. in 1/100

//Now we run -alpha- and generate mean scales:
alpha bg2cost1 bg2cost2 bg2cost3 bg2cost4, asis g(v1) // a)
alpha bg2cost1 bg2cost2 bg2cost3 bg2cost4, asis min(1) g(v2) // b)
alpha bg2cost1 bg2cost2 bg2cost3 bg2cost4, asis casewise g(v3) // c)

//-alpha-'s default is to average over items when at least one item
//has non-missing values (see a) and b) above). If you want a listwise
//deletion, i.e. only take the mean in case all variables have non-
//missing values, you have to make use of the option -casewise- (c).
//So a) and b) yield mean scores for n=568 cases, whereas c)
//yields means for only n=468, just as expected:

sum v*


    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
          v1 |       568   -.0005873    .6138736  -2.163204   1.703003
          v2 |       568   -.0005873    .6138736  -2.163204   1.703003
          v3 |       468   -.0092219    .5812742  -1.743957   1.703003

		
//Now consider the following little toy program in which we pass some stuff
//to -alpha- inside the program, let -alpha- create a temporary
//variable, and finally show a summary of the created variable:

program define foo66
version 11.2

	syntax varlist [if] [in] [, minim(integer 1) Generate(string) ]
	
	marksample touse
									
	tempvar scm
	qui alpha `varlist' if `touse', min(`minim') asis g(`scm')
	di in red `minim'
	sum `scm'
	
end

//Running the toy program with the default value of min=1, just as
//in b):
foo66 bg2cost1 bg2cost2 bg2cost3 bg2cost4

//yields means for n=468, i.e. results in a listwise deletion of the 100 cases,
//even though we told -alpha- that only one variable would be sufficient
//to calculate the mean. This is _not_ as expected:

    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
    __000001 |       468   -.0092219    .5812742  -1.743957   1.703003


//Inspecting the trace, everything seems to get picked up correctly:
//- qui alpha `varlist' if `touse', min(`minim') asis g(`scm')
//= qui alpha bg2cost1 bg2cost2 bg2cost3 bg2cost4 if __000000, min(1)
asis g(__000001)
*<snip>
//- sum `scm'
//= sum __000001

What's going on?

Cheers,

Joerg
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