We would need to see the contents of the "mplus" file that is read in. Quite possibly it's overwriting either "iter" or "count" with an incompatible data type. I would also recommend contactingWalter Leite directly to verify you have a proper mplus source file. /* partial quote follows */ The error message I keep receiving is: Read 107 items Error in count <= iter : comparison (4) is possible only for atomic and list types The code I use comes from Walter Leite's webpage: http://education.ufl.edu/leite/code/ The syntax I use is the following: #Last updated: 11/27/07 #Function (in R language) that performs item selection for the development of #short-forms of scales using an Ant Colony Optmization (ACO) Algorithm. #Author: Walter Leite, Assitant Professor, Department of Educational Psychology #University of Florida #Contact Information: walter.leite at coe.ufl.edu #The paper "Item selection for the development of short-forms of scales using #an Ant Colony Optimization algorithm" can be obtained by e-mailing the author. -- View this message in context: http://r.789695.n4.nabble.com/Ant-Colony-Optimization-Algorithm-tp4680292p4680300.html Sent from the R help mailing list archive at Nabble.com.