The quality of students' writing skills continues to concern educators. Because writing is essential to success in both college and career, poor writing can have lifelong consequences. Writing is now primarily done digitally, but students receive limited explicit instruction in digital writing. This lack of instruction means that students fail to take advantage of the affordances of digital tools. The writing process is shaped by the tools used, which makes digital writing, to an extent, different from writing with pen and paper. To better understand students' digital writing skills, we take advantage of the information provided by computer-based assessments--keyboard and mouse activity data. We examine the relationship between students' use of the keyboard and mouse during the assessment and students' writing achievement. Our data comes from the first national computer-based writing assessment in the United States, the 2011 National Assessment of Educational Progress (NAEP) assessment. Using data from over 24,100 eighth-grade students, we found that the number of keypresses had a distinct and direct effect on writing achievement scores, controlling for word count. We also identified several different patterns of keyboard and mouse activity on the computer-based NAEP assessment.