One issue that is faced when researching attrition is distinguishing between normal L2 influence on the L1 and actual attrition of the L1. Since all bilinguals experience some degree of cross linguistic influence, where the L2 interferes with the retrieval of the speaker's L1, it is difficult to determine if delays and/or mistakes in the L1 are due to attrition or caused by CLI. Also, simultaneous bilinguals may not have a language that is indistinguishable from that of a native speaker or a language where their knowledge of it is less extensive than a native speaker's; therefore testing for attrition is difficult.