Research shows that test method can exert a significant impact on test takers' performance and thereby contaminate test scores. We argue that common test method can exert the same effect as common stimuli and violate the conditional independence assumption of item response theory models because, in general, subsets of items which have a shared feature are a source of response dependence (Marais & Andrich, 2008). In this study, we use the Rasch testlet model (Wang & Wilson, 2005a) to examine the effect of test method on violating the unidimensionality assumption of the Rasch model. Results show that test formats can introduce small to large construct-irrelevant variance, contaminate test scores, and lead to the violation of the conditional independence assumption. Our findings further suggest that the degree of construct-irrelevant variance exerted by test method could be a function of test format familiarity.