Wednesday, June 25, 2014

Simple comparative effectiveness


This post shows how useful AMOS can be to test MANY structural hypotheses at once, focused on whether an outcome after the intervention is better in one vs. another group, controlling for baseline differences (a Comparative Effectiveness line of questioning).
The simple idea is that you can conclude that the 2 post-intervention means are different by comparing a 2-group (intervention vs comparison) model where the 2 post-intervention means are equal to the similar model in which the 2 means (technically the intercepts however) are different: if the 'equal intercepts' model fits worse, the means cannot be deemed equal; quite basic.
Now, there are about 15 'baseline' models one can start with to test this 'equal intercepts' second model, and hence 15 such potential conclusions; we should however use only 1 to conclude whether there is/not a difference: this is the test of differences (labeled T) conducted using the best fitting baseline model, but also considering the power of that model to really detect this difference. The problem is: a WELL fitting model may be under-powered to detect a specific effect, which happened here too.
Take a look, try it for yourself, with Excel only and the [previously] free AMOS v.5 software; I can email you these files (ask at comanus@netscape.net ), but also posted here: http://bit.ly/2eGSxlq
AMOS does MODEL COMPARISONS" for ALL possible nested models, assuming one correct and testing the other ones nested in it... Pretty cool!!!

The little paper telling this story is available at:
http://digitalcommons.wayne.edu/jmasm/vol13/iss1/6/
Coman, E. N., Iordache, E., Dierker, L., Fifield, J., Schensul, J. J., Suggs, S., & Barbour, R. (2014). Statistical power of alternative structural models for comparative effectiveness research: advantages of modeling unreliability. Journal of Modern Applied Statistical Methods, 13(1).

Friday, May 9, 2014

Here are the original 16 models published by Sewall Wright in 1921 in 'Journal of agricultural research'. Enjoy!

PS: He wasn't a 'statistician'... his title read: "Senior Animal Husbandman in Animal Genetics, Bureau of Animal Industry, United States Department of Agriculture" !

Wright, S. (1921). Correlation and causation. Part I Method of path coefficients. Journal of agricultural research, 20(7), 557-585. [CITED BY  2119, GOOGLE.SCOLAR].



COM ]

Tuesday, February 18, 2014

Late plug for Duncan's 1975 book

I had many questions about the mechanics of structural modeling, particularly because as a physicist I am always trying to figure out 1st what's 'given' and what do we need to obtain/get/calculate/estimate.
I read recently in Les Hayduk's 1996 book (p. 15-16, e.g.) that Duncan has proposed a very simple and elegant procedure for obtaining model predicted variances and covariances from the structural model parameters, which is probably even easier to follow/apply than Wright's tracing rules (see a great modernized tutorial on Phillip Wood's web pages: standardized and unstandardized parts).
That simple rule allows you for instance to also "estimate the beta coefficient" in a simple regression by hand, like this:
y = b*x + e , then multiply by x: x*y = x*b*x + x*e, take expectations (a simple operation in fact, Duncan explains it, roughly speaking just sum up across the entire sample and divide by sample size):
E(x,y) = b*E(x,x) + E(x,e), which, if we assume that the covariance between predictor and error is zero (common), takes us to the "quick estimation" of b: b = Cov(x,y) /Var(x), a well known 'formula'.
Anyways, Duncan has shown that for a rather simple (& 'saturated') model like the one above, by doing this multiplication+taking expectations repeatedly, one can obtain/estimate the structural coefficients from variances & covariances (model predicted ones; but one can also go the other way around). This figure above shows however in this simple model what-depends-on-what; neat isn't it?
References:
Duncan, O. D. (1975). Introduction to structural equation models. New. York: Academic Press.
Hayduk, L. A. (1996). LISREL issues, debates, and strategies: Johns Hopkins University Press.

Thursday, January 9, 2014

Stacked structural models

Les Hayduk (1996, Ch. 5, 155-189) presented these special kinds of multi-group Structural Equation models, the 'stacked SEMs' with different numbers of indicators, and of course I did not understand them well... Nothing works better than trying it out for yourself (and better so in AMOS first).

So: When you don't have a variable in ONE of the groups which appears in another, you can still use the same larger multi-group model (with the 'limping' variable in!).
[see my 1st image posted here: http://evaluatehelp.blogspot.com/2013/08/placeholders.html ]

How? Les Hayduk proposes a way, simply put filling in ZEROS for the covariances of the missing ('phantom') variable with all other variables, and 1's for their variances, like so:


AMOS can run models off such Excel data (you can just save these 3 covariance matrices as different worksheets in the same file, as I did; thanks, Les for including in your book the Lisrel syntax, it helped!). NOTE: AnxFam and and Work were NOT present in the 2nd and 3rd groups, but the 3-group model includes them too.
The results I got seem to replicate Les' results, after some AMOS arm-twisting.


The morale? What appears as complicated merits a 1st reading (Hayduk's 2nd book...), there is always stuff you can grab and run with it (even if not so far at first).

PS: as suggested on Semnet one should include a covariance between the errors of the looped variables, which tests whether relevant outside influences on these 2 variables have been mitted, which in our case seems not to be the case, all 3 covariances e.g. here were NonSignificant (NS) (adding such a covariance between L2 and L3 makes the model non-identified, why? well that's why the chapter merits reading...).

Hayduk, L. A. (1996). LISREL issues, debates, and strategies: Johns Hopkins University Press.