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From |
Joerg Luedicke <joerg.luedicke@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Comparing two response variables |

Date |
Tue, 1 Mar 2011 00:18:20 -0500 |

On Mon, Feb 28, 2011 at 10:32 PM, Debs Majumdar <debs_stata@yahoo.com> wrote: > The data is collected from 10000 students who had answered questions (ordinal > items) regarding their teachers (500). There were two total scores computed out > of their responses, one with 5 items and one with 7 (same 5 + additional 2). I > have been told that this new one (total score out of the 7 items) makes more > sense qualitatively. I just want to figure out if one can quantify this. > So you want to measure a latent variable using 5 or 7 indicator items (indicators in the sense of factor indicators, not dummies) and you are wondering whether one version is doing a "better" job in capturing the phenomenon of interest, right? This is usually know as the question of construct validity. Assuming that this is the first time that this particular instrument is used, there is not much you can do regarding validity. What you can check is stuff like internal reliability. I would first look at the inter-item correlations: are the 2 extra items highly correlated with the other 5? How does the correlation matrix look like? You could then indeed check, as Richard already suggested, if the alpha is different between the 5 and 7-item version. You could also fit confirmatory factor analyses for the 2 versions, preferably with varying intercepts across teachers, and see whether there are differences with respect to model fit. If the 5-item and 7-item version would perform equally well with respect to all of that, you could just use the 7-item version if that is slightly preferred by theoretical intuition. The same would of course apply if the 7-item version would perform better. The only problem could be if the 5-item version shows better reliability but theoretically it would make more sense to take the 7-item version, and that is because the instrument's reliability does not inform you about its validity: Whatever is measured by the 5-items is measured well, only that you cannot be sure what exactly it is that was measured. Or, in other words, the instrument is only measuring a part of what it was supposed to measure, but this part is measured well. J. * * 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: Comparing two response variables***From:*Debs Majumdar <debs_stata@yahoo.com>

**Re: st: Comparing two response variables***From:*Joerg Luedicke <joerg.luedicke@gmail.com>

**Re: st: Comparing two response variables***From:*Debs Majumdar <debs_stata@yahoo.com>

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