The simple view of writing suggests that written composition results from oral language, transcription (e.g., spelling/handwriting), and self-regulation skills, coordinated within working memory. The model provides a number of implications for the interpretation of psychoeducational achievement batteries. For instance, it hypothesizes that writing skills are only partially related to each other through a hierarchy of levels of language (e.g., subword, word, sentence, discourse levels) and that transcription skills such as spelling mediate the effects of language skills on composition. We evaluated implications of the simple view of writing in the Wechsler Individual Achievement Test, 3rd Edition (WIAT-III). Using structural equation modeling, we established that WIAT-III writing tasks are only partially related to each other within both the battery's normative sample and an independent sample of students referred for special education. We also described how lower level writing skills mediated the effects of language skills on higher level writing skills. However, these effects varied across normative and referral samples.