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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 08:59:07 +0000 |

-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 >> * >> * 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/ >> > * > * 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/ * * 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/

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

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

**Re: st: -alpha-, unexpected behavior***From:*Barth Riley <barthriley@comcast.net>

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