[R] Factor Analysis in MASS4

[John Fox]

Dear Kevin,

At 09:55 PM 8/29/2002 +1200, Ko-Kang Kevin Wang wrote:

>I had a look at the MASS4 scripts in the MASS package, in Ch 11.3 Factor
>Analysis, there is a section of codes like:
>
>   data(ability.cov)
>
>   ability.FA <- factanal(covmat = ability.cov, factors = 1)
>   ability.FA
>   (ability.FA <- update(ability.FA, factors = 2))
>   #summary(ability.FA)
>   round(loadings(ability.FA) %*% t(loadings(ability.FA)) +
>              diag(ability.FA$uniq), 3)
>
>Unfortunately I still haven't received the book I ordered, so I can't look
>this up.

I, too (and I'm sure many others) are also waiting.

>Two questions:
>
>1) What does the update() do?  I mean, what happens if I replace it with
>      factanal(covmat = ability.cov, factors = 2)

As is generally the case, update methods allow you to respecify a model, 
here a factor analysis, changing some arguments to the modeling function. 
As you probably discovered, factanal(covmat = ability.cov, factors = 2) is 
equivalent.

>2) What does the last command, the formulae in round() mean?  I tried it
>and it produced a matrix that looks kind of like correlation matrix of
>some sort...

These are the reproduced correlations, which may be compared with the 
observed correlations, round(ability.FA$correlation, 3).

I hope that this helps,
  John
-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox
-----------------------------------------------------

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