Writing well is a major cognitive challenge, because it is at once a test of
memory, language, and thinking ability. It demands rapid retrieval of domain-specific
knowledge about the topic from long-term memory (Kellogg, 2001). A high degree of
verbal ability is necessary to generate cohesive text that clearly expresses the ideational
content (McCutchen, 1984). Writing ability further depends on the ability to think clearly
about substantive matters (Nickerson, Perkins, & Smith, 1985).