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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 >> * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: -alpha-, unexpected behavior***From:*Joerg Luedicke <joerg.luedicke@gmail.com>

**Re: st: -alpha-, unexpected behavior***From:*Nick Cox <njcoxstata@gmail.com>

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