2006). As a result, early childhood education policy makers often face the challenge of implementing a balanced teacher qualification standard that helps recruit teachers who can produce high-quality child development outcomes but also ensures that teacher salaries are affordable.

However, prior research on the link between teacher characteristics in early childhood programs and children’s developmental outcomes is not uniformly positive. Some studies show that classrooms with more highly educated and trained teachers are associated with higher quality care (Burchinal et al., 2002; Phillips et al., 2001). In contrast, other studies have demonstrated that once unobserved differences across centers are controlled for using center fixed effects, the effect of teacher’s education and training on child development outcomes disappear (Blau, 2000; Early et al., 2006, Early et al., 2007). These mixed findings on teacher characteristics are not unique to preschools; similar research findings are obtained from non-experimental research on primary and secondary school teachers (Early et al., 2007; Hanushek and Rivkin, 2006). Evidence from developing countries shows slightly stronger support for a positive relationship between teacher education and student achievement (Hanushek, 2003; Hanushek and Rivkin, 2006). However, one criticism of many of these studies is that they do not adequately control for omitted variables. Further analyses using extensive control variables—especially from low- and middle-income country contexts—can help clarify the inconclusive evidence that exists on this topic.

# _Structural characteristics_

There are other characteristics of preschools that can perhaps be regulated even more easily than teacher characteristics. These so-called structural characteristics are usually easier to mandate and less costly to assess than observing the quality of individual classrooms. As a result, there is considerable policy interest in understanding whether structural characteristics (beyond teacher qualifications) matter in explaining children’s early development.

Two structural characteristics that are often investigated in studies of preschool quality are teacher-child ratio and hours of operation. For teacher-child ratio, studies have generally shown that in lower-ratio settings, teachers spend less time managing children in the classroom and are able to provide more stimulating, responsive, and warm care (Phillips et al. 2001, Burchinal et al., 2000). However, Blau (2000), Mashburn et al. (2008) and Duncan and Magnuson (2013) find that the significant association between teacher-child ratio and child development outcomes disappears when regression models control for other characteristics of the preschool classroom (i.e., adding more classroom-level variables or using classroom fixed effects). This suggests that while teacherchild ratios matter, they may only affect children’s development indirectly, by influencing their experiences with their teachers and peers within the classroom.

Another structural characteristic that has received attention in the literature is dose or duration of preschool, which is often measured by hours of operation. To date, findings on the relationship between hours spent in early childhood education and children’s developmental outcomes are mixed. For cognitive and language development, studies find positive correlation between time spent in care and children’s development but the association disappears once children’s family background and other preschool aspects are controlled (Duncan, 2003; NICHD 2000; Vandell, 2004). Similarly, studies that have looked at children’s social competence and behavioral problems have also found mixed results. In the US, researchers found that even when holding family factors constant, children who spend more hours in preschool have more behavioral problems than their peers who spend fewer hours in preschool settings (NICHD, 2003; Vandell, 2004). In contrast, more recent research from Norway finds little evidence of behavioral issues in children from spending more hours in child care (Zachrisson et al, 2013). To our knowledge, early 4