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From |
Joerg Luedicke <[email protected]> |

To |
[email protected] |

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 * * 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:*Nick Cox <[email protected]>

**Re: st: -alpha-, unexpected behavior***From:*Barth Riley <[email protected]>

**Re: st: -alpha-, unexpected behavior***From:*Joerg Luedicke <[email protected]>

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