Re: st: -alpha-, unexpected behavior

[Nick Cox <njcoxstata@gmail.com>]

-marksample- still has the important role of paying attention to -if-
and -in- when specified.

On Fri, Mar 23, 2012 at 5:08 AM, Barth Riley <barthriley@comcast.net> wrote:
> According to the marksample help documentation:
>
>    5.  The marker variable is set to 0 in observations for which any of the
>        numeric variables in varlist contain a numeric missing value.
>
>
> So my interpretation is that if any of the variables in varlist of your
> program have a numeric missing value, marker is set to 0 regardless of the
> value of minim. I am not sure what purpose the marksample command provides
> in your code--what happens if you were to remove it?
>
> Barth
>
>
>
> On 3/22/2012 11:10 PM, Joerg Luedicke wrote:
>>
>> 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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