In 2025, Leanlab Education partnered with nonprofit writing and grammar platform provider Quill.org to examine the impact of the platform on student writing skill development. Quill is a free, nonprofit, digital writing platform focusing on sentence-level skill development, providing brief, targeted activities with immediate feedback to supplement classroom instruction. To understand the platform's effectiveness at scale, Quill enlisted Leanlab to conduct a large-scale, post-hoc study tracking outcomes for over 100,000 students across 2,600 schools during the 2023-2024 school year. The question: Does consistent use of Quill translate into measurable improvements in student writing skills?
The Research
Leanlab examined data from 100,200 students in 2,600 public and charter schools across the United States. The study included diverse educational settings—urban, suburban, and rural—with students primarily in grades 4-12, with 88% in grades 6-8. To understand impact across different socioeconomic contexts, the researchers compared outcomes for students in Title I schools (which serve higher proportions of students from low-income families) with those in non-Title I schools.
Quill’s digital writing platform includes six tools designed to support grammar, syntax, and sentence construction; this study focused on one of those tools, Quill Diagnostics. The quasi-experimental study drew on archival data collected through the Quill platform. All students completed Quill's Baseline Diagnostic, and based on their results, students received recommendations for targeted practice activities. Researchers then compared two groups: students who completed at least one recommended practice activity (the intervention group) and students who received recommendations but didn't complete any practice activities (the comparison group).
After using Quill, all students completed a Growth Diagnostic to measure their progress. The Growth Diagnostic assesses sentence-level writing skills, such as subject-verb agreement, compound structures, and commonly confused words. The study used advanced statistical modeling to account for baseline scores and school-level grouping, providing a rigorous assessment of Quill's impact.
The Findings
The analysis found that Quill produces meaningful improvements in student writing skills, with benefits that are both statistically significant and educationally meaningful according to established benchmarks.
1.6x Faster Growth with Practice
Students who practiced using Quill showed substantially greater improvement than those who didn't. The intervention group gained 18.3 percentage points on the Growth Diagnostic, while the comparison group gained 11.5 points—a difference of 6.8 percentage points or 1.6x rate of growth.
The effect size falls well within what education researcher John Hattie calls the "zone of desired effects"—improvements that are large enough to be considered educationally meaningful, not just the result of typical development or general teaching.
More Practice, Greater Gains
The study findings indicated a strong dose-response relationship: the more students practiced, the greater their growth. Researchers divided students into three groups based on how many activities they completed:
Partial Intervention (1-9 activities): Students improved by 10.9 percentage points
Full Intervention (10-19 activities): Students improved by 15.7 percentage points
Extended Intervention (20+ activities): Students improved by 20.3 percentage points
All three groups showed gains in Hattie's "zone of desired effects," but the extended intervention group's results were particularly strong. These findings suggest that even moderate use of Quill can produce meaningful results, while sustained engagement can drive even greater improvement.
Narrowing the Achievement Gap
Perhaps most encouragingly, the study found that Quill helped narrow achievement gaps between students in Title I and non-Title I schools. Students in Title I schools improved by 38% from baseline, while students in non-Title I schools improved by 24%.
Both groups showed strong growth, but the relative impact compared to their respective comparison groups was slightly higher in Title I settings (effect size of 0.53 versus 0.43). This suggests that while Quill is effective across varied contexts, it may be a particularly valuable intervention in schools serving economically disadvantaged students.
A detailed look at the study methods, sample, and limitations, along with further analysis of the findings, is available in the full report.
Looking Ahead: Building the Future of AI-Powered Writing Tools
Building on a shared commitment to education research and evidence-based product development, Quill and Leanlab Education are embarking on an exciting new chapter of partnership that addresses one of education's most pressing challenges: ensuring that AI-powered educational tools deliver high-quality, trustworthy feedback to students.
As part of a $2.8 million initiative funded by Learning Commons, Quill and Leanlab are developing a research protocol and creating a large, public dataset featuring teacher feedback on student writing samples. This dataset will be annotated to highlight effective feedback practices rooted in learning science, creating a benchmark against which AI tools can be evaluated and improved.
The goal is to create open, public infrastructure that developers can use to both assess current AI tools and train future ones to provide feedback aligned with research-backed strategies for strengthening student writing. This work promises a future where high-quality writing support is available at scale—providing the kind of detailed, personalized feedback that helps learners grow.
To learn more about Leanlab Education's independent research services or to explore partnership opportunities, visit our Codesign Product Research webpage. To learn more about Quill's approach to writing instruction and the full results of this study, visit Quill.org.