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From |
"Gramig, Benjamin M" <bgramig@purdue.edu> |

To |
"'Joseph Coveney'" <jcoveney@bigplanet.com>, "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu> |

Subject |
st: RE: Why do Stata Cronbach's Alpha values not match SAS? |

Date |
Thu, 28 Apr 2011 12:43:37 -0400 |

Thank you Joseph. The suggestion to walk through a worked example like the one contained in the SAS manual is a very good one. I didn't have any errors in my data statements in SAS, but by walking through this process and thinking about the other things you mentioned I have been able to determine why I was seeing such big differences. The key difference is in how SAS handles variables with negative signs (correlations) when compared to Stata. Stata automatically reverses the coding of the variables with opposite signs from the group, whereas SAS does not. Using the -alpha- command in Stata with the -, item asis- options will give you the same output as the SAS command -PROC CORR...alpha-. SAS's -nomiss- option works equivalent to Stata's -, casewise- option. SAS doesn't appear to have an option that automatically reverses the sign for negatively correlated variables, so I guess I would need to recode the variables manually in my data to calculate the alpha reliability statistic for the scale equivalently in SAS. I feel I still need to use SAS for the accompanying PCA of the items in the scale because I cannot figure out how to output factor loadings or apply rotations following -polychoric pca-. It is my understanding that for ordinal/Likert data I need to use polychoric correlations (or another technique) to perform PCA correctly. Many thanks, Ben Gramig -----Original Message----- From: Joseph Coveney [mailto:jcoveney@bigplanet.com] Sent: Thursday, April 28, 2011 12:23 AM To: statalist@hsphsun2.harvard.edu Cc: Gramig, Benjamin M Subject: Re: Why do Stata Cronbach's Alpha values not match SAS? Benjamin Gramig wrote: I have used the alpha, item- command in Stata to calculate Cronbachs alpha to evaluate scale reliability of a group of Likert scale survey questions and received the following Stata output: [omitted] When I attempt to use SAS to calculate the same Alpha reliability value using the -PROC CORR data alpha- command I get much lower values (pasted below) for items in the scale as well as the overall alpha for the entire scale. The qualitative results in terms of sign and relative magnitude of alphas calculated when removing individual items from the scale are consistent with Stata, but not the magnitudes. I have read the manuals for both pieces of software and it is not clear to me that there are differences in what is being reported by both software packages. Has anyone else encountered this drastic difference? I assume that there is something systematically different about what is going on in the two packages to calculate the reported values, but I couldn't determine what this difference was. It should be noted that I turned to SAS to be able to use polychoric correlations in a PCA with a full set of diagnostics, outputs and rotations available. This seems necessary for my ordinal data, despite ignoring this in the comparison of alpha calculations shared here. I did explore -polychoricpca- in Stata before deciding to use SAS. Any insights are greatly appreciated, [omitted] -------------------------------------------------------------------------------- As far as insights for the differences between software, my first suspicion is that you're analyzing different data. That DATA step can be a tricky thing if you're accustomed to Stata, and conversely those who are at-home with SAS's way of going about data management can be tripped up by trying to emulate it in Stata. A second insight based upon experience: when in doubt, compare results with a worked example. Choosing an example from SAS and running it in Stata (see below) shows that the two packages will give the same results when fed the same dataset and options (in this case, listwise deletion of observations with at least one missing value). There is one feature not explored in the worked example below: I don't know how SAS handles sign flips as compared to Stata's -alpha-. The worked example below doesn't have any flipped scales and, as you mentioned, your data do. So, that's something to look into further if you can find a worked example that has reversed-sense variables. As far as your choosing SAS over Stata for principal component analysis with categorical data, I don't have any insight. But I have a few comments: 1. I assume principal component analysis is a considered decision--that it's what you really want to do. 2. You can do any rotation in Stata that I'm aware of, at least for exploratory factor analysis. 3. Based upon my experience with M-Plus, at least a couple of the popular (must-show) diagnostic tests appear not to work well with categorical variables. 4. At least for exploratory factor analysis, I didn't notice any practical gain (can-do-science-better) in moving away from Stata because of categorical variables. Joseph Coveney version 11.2 clear * set more off input float Weight double (Length3 HtPct WidthPct) /* omitted: get the dataset (and the SAS output to compare Stata's) from http://support.sas.com/documentation/cdl/en/procstat/63104/HTML/default/viewer.h tm#procstat_corr_sect032.htm */ end generate double Weight3 = Weight^(1/3) generate double Height = HtPct * Length3 / 100 generate double Width = WidthPct * Length3 / 100 drop HtPct WidthPct label data "Fish Measurement Data" generate byte listwise_missing = 0 foreach var of varlist Weight3 Length3 Height Width { replace listwise_missing = 1 if mi(`var') } summarize Weight3 Length3 Height Width if !listwise_missing pwcorr Weight3 Length3 Height Width, casewise // NOMISS alpha Weight3 Length3 Height Width, casewise item alpha Weight3 Length3 Height Width, casewise item std exit * * 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**:**st: Re: Re: Why do Stata Cronbach's Alpha values not match SAS?***From:*"Joseph Coveney" <jcoveney@bigplanet.com>

**References**:**st: Why do Stata Cronbach's Alpha values not match SAS?***From:*"Gramig, Benjamin M" <bgramig@purdue.edu>

**st: Re: Why do Stata Cronbach's Alpha values not match SAS?***From:*"Joseph Coveney" <jcoveney@bigplanet.com>

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