The goal of Hawaii’s “preschool to third grade” initiative was for every child in the state to read at grade level by third-grade.

An ambitious goal? Definitely.

We recently completed a five-year evaluation of the initiative and found that third-grade reading scores significantly improved. These gains are equivalent to nine additional weeks of school over the same period.

The initiative was motivated by research demonstrating that children who enter school behind in learning skills tend to stay behind academically. Advocates of this “preschool to third grade” reform — called “P-3” for short — argue that promoting strong skills at kindergarten entry, a smooth transition to kindergarten, and the alignment of curriculum and learning approaches from preschool through third grade will help students succeed.

Despite widespread implementation around the country, the P-3 approach had not been rigorously evaluated. The RAND Corporation’s study was the first to analyze whether a P-3 initiative improved third-grade reading scores.

We relied on data from more than 190 elementary schools and nearly 99,000 students between 2008 and 2014. We found that the more years students spent participating in the initiative, the more likely their reading scores were to have improved by third grade.

The initiative targeted schools in five areas where reading scores were lower than the state average. In 2008, the year before the initiative began, third graders in P-3 schools were 11 percentage points less likely to be proficient in reading than those in non P-3 schools. As of 2014 — the program’s seven-year mark — the Hawaii state reading assessment showed that the gap in reading proficiency between P-3 schools and those that had not been exposed to the initiative had narrowed by 4 percentage points.

These gains are both statistically and educationally significant. It is hard to improve children’s basic skills. In comparison, these improvements are greater than those found for many other education reform efforts aimed at improving elementary school student performance.

What helped? Written commitments to collaborate from Hawaii Department of Education and pre-kindergarten leaders in each site were explicitly required as a condition of funding by Hawaii P-20 Partnerships for Education, an organization of pre-K to higher education leaders that oversaw the implementation of the initiative. Their collaboration became a pivotal component of the initiative’s success. Cooperative, ongoing relationships between local pre-K and kindergarten to third-grade leaders and practitioners also were established and flourished.

Individual sites effectively tailored activities to local needs and resources. Other aspects that made a critical difference included exposing key policymakers and practitioners to professional development on early childhood education. New state-level early learning standards developed as part of the initiative helped local partners align pre-K and K-3 curricula and expectations for children.

Like many great ideas, some of the changes that P-3 sites worked to implement may seem obvious in retrospect — aligning curriculum across the early grades, improving access to early-learning support, getting more children enrolled in preschool, making the transition to kindergarten more seamless.

But many of these efforts would not have happened without the initiative.

Our findings provide strong evidence that the P-3 approach is a promising education reform that should be considered by those seeking to improve early education performance.