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Re: st: -alpha-, unexpected behavior


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: -alpha-, unexpected behavior
Date   Fri, 23 Mar 2012 08:57:32 +0000

Sorry, I didn't scroll down far enough to see the answers.

On Fri, Mar 23, 2012 at 8:56 AM, Nick Cox <njcoxstata@gmail.com> wrote:
> This question is a model question, explaining the problem clearly,
> concisely, precisely and reproducibly!
>
> Don't blame -alpha-. What is different inside your program is that
> -marksample- is doing its job, including marking out observations with
> missing values. -alpha- never gets allowed to exercise its indulgence.
>
> For your purposes you want
>
> marksample touse, novarlist
>
> Nick
>
> On Fri, Mar 23, 2012 at 4:10 AM, Joerg Luedicke
> <joerg.luedicke@gmail.com> wrote:
>
>> 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
>>

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