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I have to run a factor analysis on a dataset made up of dichotomous variables (0=yes, 1= no) and I don´t know if I'm on the right track.

Using tetrachoric() I create a correlation matrix, on which I run fa(data,factors=1). The result is quite near to the results i receive when using MixFactor, but it´s not the same.

  1. Is this ok or would you recommend another procedure?
  2. Why does fa() work and factanal() produce an error? (Fehler in solve.default(cv) : System ist für den Rechner singulär: reziproke Konditionszahl = 4.22612e-18)
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    $\begingroup$ I don't speak (German?) but the error looks like it's due to the tetrachoric matrix being singular (non-invertible). Even with a good sized sample some estimates of polychoric correlation matrices can fail to be proper correlation matrices. My hunch is that fa() is using principal components and factanal() is doing maxmimum likelihood FA, but that's not obvious to me from the documentation for fa(). $\endgroup$ – JMS May 30 '11 at 15:59
  • $\begingroup$ @cada How many items/subjects do you have? And what method is implemented in MixFactor? $\endgroup$ – chl May 30 '11 at 15:59
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    $\begingroup$ @cada why are you running the factor analysis? If you are aiming to estimate participant ability, IRT may be a better approach. on the matter of fa versus factanal, fa uses minimum residual by default, while factanal uses an ML method. $\endgroup$ – richiemorrisroe May 30 '11 at 17:43
  • $\begingroup$ @chl: way too few! I just started the survey and by now there are just 45 subjects. I cant collect new data in teh moment and that´s why I try to do as much of the r code as possible... $\endgroup$ – cada May 31 '11 at 7:58
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    $\begingroup$ @cada A CFA with 45 subjects would be surrealist :-) I'll try to add my thoughts later. $\endgroup$ – chl May 31 '11 at 11:47
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To sum up, with n=45 subjects you're left with correlation-based and multivariate descriptive approaches. However, since this questionnaire is supposed to be unidimensional, this always is a good start.

What I would do:

  • Compute pairwise correlations for your 22 items; report the range and the median -- this will give an indication of the relative consistency of observed items responses (correlations above 0.3 are generally thought of as indicative of good convergent validity, but of course the precision of this estimate depends on the sample size); an alternative way to study the internal consistency of the questionnaire would be to compute Cronbach's alpha, although with n=45 the associated confidence interval (use bootstrap for that) will be relatively large.
  • Compute point-biserial correlation between items and the summated scale score; it will give you an idea of the discriminative power of each item (like loadings in FA), where values above 0.3 are indicative of a satisfactory relationship between each item and their corresponding scale.
  • Use a PCA to summarize the correlation matrix (it yields an equivalent interpretation to what would be obtained from a multiple correspondence analysis in case of dichotomously scored items). If your instrument behaves as a unidimensional scale for your sample, you should observe a dominant axis of variation (as reflected by the first eigenvalue).

Should you want to use R, you will find useful function in the ltm and psych package; browse the CRAN Psychometrics Task View for more packages. In case you get 100 subjects, you can try some CFA or SEM analysis with bootstrap confidence interval. (Bear in mind that loadings should be very large to consider there's a significant correlation between any item and its factor, since it should be at least two times the standard error of a reliable correlation coefficient, $2(1-r^2)/\sqrt(n)$.)

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  • $\begingroup$ Thank you very much! Your answer is so detailed and really really helpful! Thank you! $\endgroup$ – cada Jun 7 '11 at 9:43
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This thread has a good Google position for the "System ist für den Rechner singulär: reziproke Konditionszahl" error using factanal (in English: "system is computationally singular: reciprocal condition number") - therefore I shall add a comment:

When the correlation matrix is calculated a priori (e.g., to pairwisely delete missing values), make sure that factanal() does not think that the matrix is the data to analze (https://stat.ethz.ch/pipermail/r-help/2007-October/142567.html).

PREVIOUS: matrix = cor(data, use="pairwise.complete.obs")  # For example
WRONG: factanal(matrix, 3, rotation="varimax")
RIGHT: factanal(covmat=matrix, factors=3, rotation="varimax")

BurninLeo

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