[R] start.val in GRM (ltm package)

Jeff r at jp.pair.com
Thu Jul 19 20:38:04 CEST 2012

   Can someone please give me an example of how to enter starting values for a
   GRM (IRT) model using the ltm package?
   The instructions from the ltm manual are below, but when I create either a
   list of the values or a matrix I get the error, "start.val not of proper
   type."  I can find no example on the web other than using NULL or Random,
   which won't work for me.
   The code I'm using looks something like this:
   row1 <- c(0.146,     1.505,    2.158)
   row2 <- c(4.124,     5.790,    1.589)
   row3 <- c(3.635,     4.916,    1.426)
   startval_list <- c( row1 , row2 , row3)
   Results <- grm( DelT4 , constrained = FALSE, start.val = startval_list)
   ...which produces the error mentioned above.
   Manual instructions:
   start.val -
   a list of starting values or the character string "random" . If a list, each
   one of
   its elements corresponds to each item and should contain a numeric vector
   initial values for the extremity parameters and discrimination parameter;
   even if
   constrained = TRUE the discrimination parameter should be provided for all
   the items. If "random" random starting values are computed.

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