[R] about MCMC pack

[Spencer Graves]

	  I'm not able to replicate the error.  The first example in the 
"MCMCirtKd" help file worked fine for me.  It ran in under 4 minutes on 
my computer.  The "plot" produced several pages of output, which 
required manual intervention to move from one to the next.  The 
'summary' command also seemed to perform appropriately, without an error 
message.  I killed the second example after about 15 minutes of compute 
time, so I don't know if it would have worked or not.  You might get 
better help if you provide a simple, self-contained example that 
actually replicates the error message you got (as suggested in the 
posting guide, www.R-project.org/posting-guide.html).

	  What have you tried to simplify the problem?  For example, can you 
find numbers smaller than 'burnin = 1000, mcmc = 10000' for which you 
get the same problem, e.g., "burnin=2, mcmc=2" or 'burnin=100, 
mcmc=100'?  If yes, that should substantially reduce the time required 
to isolate this problem.

	  Also, if you change some of the arguments, when do you get the 
problem and when does it disappear?  I noticed you specified 'thin=1, 
verbose = 1, seed = NA,alphabeta.start = NA, b0 = 0, B0=0, store.item = 
FALSE, store.ability=FALSE, drop.constant.items=TRUE'.  All but one of 
these is at its default value.  In R, arguments assuming their default 
values do not have to be provided.  If dropping one of these arguments 
changes the behavior of the function, that's a problem.

	  In particular, what do you get with the default value for 
'item.constraints'?  If the problem goes away, that suggests you need to 
be more careful about specifying 'item.constraints'.

	  If this fails, my next step is 'debug':  You can make local copies of 
MCMCirtKd (and MCMCordfactanal, which it calls to do most of the work) 
and follow along, step by step, until you find where it bombs -- and 
hopefully also in the process figure out what to change to fix it.

	  hope this helps.
	  Spencer Graves
p.s.  Please also include 'sessionInfo()';  otherwise, potential 
respondents don't know if you are using an obsolete version of something 
  that might already have been fixed.  My sessionInfo() is as follows:

Version 2.3.0 (2006-04-24)
i386-pc-mingw32

attached base packages:
[1] "methods"   "stats"     "graphics"  "grDevices" "utils"     "datasets"
[7] "base"

other attached packages:
MCMCpack     MASS     coda  lattice
  "0.7-1" "7.2-27" "0.10-5" "0.13-8" 	

Mariagiulia Matteucci wrote:
> Hello,
> I tryed to use the MCMC pack, particularly the function MCMCirtKd to
> simulate the posterior distribution in a multidimensional IRT model.
> The code I used is:
> 
> posterior1 <- MCMCirtKd(Y, dimensions=2,
> item.constraints=list("V2"=list(3,0)),
>    burnin = 1000, mcmc = 10000, thin=1, verbose = 1, seed = NA,
>    alphabeta.start = NA, b0 = 0, B0=0, store.item = FALSE,
>    store.ability=FALSE, drop.constant.items=TRUE)  
> 
> 
> where Y is a matrix subjects x items (1500 x 32) of simulated response
> sequences.
> 
> After running the program, if I ask for a plot or summary of the MCMC
> object "posterior1", I obtain this message:
> 
> 
> Error in as.matrix.mcmc(x) : the lenght of'dimnames' [2] doesn't
> coincide with the array extension
> More: Warning message:
> the number of elements to replace is not a multiple of the lenght of
> replacement
> 
> 
> I cannot interpret this warning message, thank you for help or
> suggestions!
> 
> Mariagiulia
> 
> 
> 
> 
> Mariagiulia Matteucci
> Dipartimento di Scienze Statistiche “Paolo Fortunati”
> Università di Bologna
> Via Belle Arti, 41 
> 40126 Bologna (ITALY)
> e-mail: matteucci at stat.unibo.it
> TEL: +39 051 272503
> FAX: +39 051 232153
> 
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