Our objective was to investigate the impact of the Science Writing Heuristic (SWH) on undergraduates' ability to express logical conclusions and include appropriate evidence in formal writing assignments. Students in three laboratory sections were randomly allocated to the SWH treatment ("n"?=?51 students) with another three sections serving as a control ("n"?=?47 students). All sections received an identical formal writing assignment to report results of laboratory activities. Four blinded raters used a 6-point rating scheme to evaluate the quality of students' writing performance. Raters' independent scoring agreement was evaluated using Cronbach's a. Paper scores were compared using a "t"-test, then papers were combined into low-scoring (3.5 of 6 points) or high-scoring (>3.5 of 6 points) sets and SWH and control cohorts were compared using Pearson's chi-square test. Papers from the SWH cohort were significantly ("P"?=?0.02) more likely to receive a high score than those from the control cohort. Overall scores of SWH cohort papers tended to be higher ("P"?=?0.07) than those from the control cohort. Gains in student conceptual understanding elicited by the SWH approach improved student ability to express logical conclusions about their data and include appropriate evidence to support those conclusions in formal research reports. Extending the writing tasks of the SWH to formal writing assignments can improve the ability of undergraduates to argue effectively for their research findings.