Simple and complex addition problems were presented for true/false verification to 22 subjects across two times of measurement to test the general model for simple and complex addition proposed by Widaman, Cormier, & Geary (1985). Models fit to average RT data revealed that subjects were processing complex problems columnwise, beginning with the units column. Column sums seemed to be obtained through an incrementing process, and subjects exited problems as soon as a colimn error was encountered. Group level models were the same across complex problem types and for both times of measurement. However, individual level analyses suggested that nearly half of the subjects used a different processing strategy to obtain column sums for the second time of measurement. Results support the multi-staged model proposed by Widaman et al. (1985), but individual level results suggest that information processing models developed from group data may not represent the processing strategies used by all subjects, or the same subjects at different times.