This study examined whether or not fourth-graders could fully demonstrate their writing skills on the computer and factors associated with their performance on the National Assessment of Educational Progress (NAEP) computer-based writing assessment. The results suggest that high-performing fourth-graders (those who scored in the upper 20 percent in the computer- and paper-based writing assessments on the NAEP) write better on the computer than on paper and can fully demonstrate their writing proficiency in a computer-based assessment. There are, however, indications that low-performing fourth-graders (i.e., those in the bottom 20 percent) and middle-performing fourth-graders (representing the middle 60 percent) may have performed less well on the computer than on paper. This suggests a potential differential effect of writing on the computer on writing performance that might have contributed to the widened achievement gap between high- and non-high-performing (i.e., low- and middle-performing) students in the 2012 computer-based pilot assessment, compared to the 2010 paper-based pilot assessment. Unequal prior exposure to writing on the computer and preference for mode of writing (paper vs. computer) appear to be associated with this widening of the achievement gap. For example, about 94 percent of high-performing students in 2012 reported having access to the Internet at home, compared to about half (52 percent) of low-performing students. The score difference between those with access to the Internet at home and those without was substantive, with an effect size of 0.87. The statistics presented in this report are estimates based on a NAEP pilot assessment conducted with a nationally representative sample of 10,400 grade 4 students. As a pilot assessment, the sample was not adjusted to account for nonresponse bias. As a result, the sample may not completely represent the grade 4 population. The following are appended: (1) Exact text of questions in the 2012 NAEP Writing Contextual Questionnaires; (2) Tables; (3) Tables for Chapter 3; (3) Tables for Chapter 4; (4) Tables for Chapter 5; and (5) Figures. [The American Institutes for Research provided technical and editorial services.]