IF YOU LIVE in Philadelphia and watch the news, you've probably heard about the newly approved sugary-drink tax, and you likely heard that most of this new revenue will go to fund more slots for children to attend public prekindergarten programs.

While there was much debate in Council and the press about the benefits and costs of the tax for consumers and the economy, there was little, if any, debate over the worthiness of this cause. Even at the national level, political figures ranging from Ivanka Trump to Hillary Clinton espouse the value of pre-K.

In part, that's because no other topic in education has been studied as extensively, or as rigorously, as pre-K. Beginning with the Perry Preschool project in the 1970s, we have amassed decades of evidence that unequivocally points to the importance of access to early childhood education. Studies have shown that high-quality pre-K closes nearly half the achievement gap that exists in children up to age 5. Long-lasting effects on social skills, health outcomes and later school success also have been documented.

Research is equally clear that not all early-education programs are created equal. So, as the city embarks on this effort to expand access to pre-K, it is essential that policymakers focus on supporting strong implementation and quality improvement. One resource at their disposal: the state's Quality Ratings and Improvement System, called Keystone STARS. STARS evaluates and rates early-childhood programs, and also provides resources and targeted supports to help them improve quality. STARS also is undergoing revision this year by the Pennsylvania Office of Child Development and Early Learning. This month, Research for Action released a policy brief to examine existing research on the "quality characteristics" of early-childhood education programs that improve child outcomes, and outline how a QRIS such as STARS can be revised to better align with and promote these quality characteristics.

What does "quality" mean in early-childhood education? Research suggests that responsive relationships between children and adults, strong instruction, small class sizes, meaningful family engagement, developmentally appropriate curricula and access to teachers trained to implement them all boost the impact of pre-K on child outcomes.

Of course, these things cost money. Pre-K providers face a unique dilemma in that the tuition they charge, whether paid by the family or covered by state child-care subsidy (for low-income children), does not cover the true cost of care, let alone allow for investments in infrastructure or wage increases to attract and retain highly qualified staff.

For example, a STAR 4 (high quality-rated) facility in the Philadelphia area could receive a maximum total of $9,789 for each child in pre-K - $8,489 in subsidy tuition and $1,300 in STARS-tiered reimbursement to support quality. Meanwhile, Philadelphia's Nonprofit Finance Fund estimates the average cost of care per child in a STAR 4 facility at $12,789. Providers are unable to close this gap by raising tuition rates, which would quickly price out all but the most privileged families. For this reason, early-education providers, especially those serving low-income neighborhoods, are forced to budget very differently from other small businesses, often holding limited cash balances and operating month-to-month. Directors often report using STARS supports to help cover basic operating costs, limiting the amount they can devote to robust quality improvement or staff compensation.

The result? A recent federal study found that Pennsylvania's regular education preschool teachers fall in the lowest bracket for annual median pay, earning $21,930 to $23,890. Child-care teachers serving infants and toddlers earned even less; their median pay was only $19,590 annually, well below a living wage for anyone supporting a family. Until this improves, early-education providers will lack the capacity to support and retain the strong teachers and quality programming that our children and families deserve.

Even as Keystone STARS is streamlined to align with the characteristics of program quality that matter most, experience suggests that pre-K providers will continue to struggle to provide high-quality care until policymakers invest more in what we know works. This means increasing reimbursement rates to match the true cost of care, and investing in scholarships and evidence-based coaching and professional development to grow the pipeline of credentialed, high-quality staff.

Philadelphia's new drink tax suggests we're serious about expanding pre-K. We need to be equally serious about paying for quality.

Della Jenkins is a policy analyst and Kate Shaw is executive director of Research for Action, a Philadelphia-based nonprofit education research organization. Read RFA's latest policy brief on early-childhood education, and learn more about the organization at www.researchforaction.org.