In the past decades several studies have related the amount of instructional time to student achievement following Carroll’s (1963) sensible premise that learn something is a function of the time allocated to learn it. Although some studies identified effects of learning time on student achievement, the effects are often inconsistent across grade and outcomes, and on average modest (e.g., Karweit & Slavin, 1982, Fredrick & Walberg, 1980; Scheerens, 2014; Seidel & Shavelson, 2007).
A possible explanation for the mixed findings on the effects of instructional time is that many studies are based on relatively small and not representative samples. Another limitation of the research in this area is that most studies have been conducted in the US. To address these limitation, Baker, Fabrega, Galindo, and Mishook (2004) analyzes the representative samples of secondary school student from three international large-scale assessments (PISA, TIMSS, CIVIC) that have been conducted in 28 to 38 countries. They used the cross-sectional variation to estimate the association between instructional time and student achievement. The study basically replicated the inconsistent findings from previous research, in some countries the associations were statistically significant positive, in others they were statistically negative, and often non-significant correlations were observed. The scope of the study is limited, though, because selection effects may have biased the cross-sectional estimates of the association between instructional time and achievement.
In a recent study, Lavy (2015) proposed an alternative approach to identify the effect of instructional time that can takes possible selection bias into account when analyzing cross-sectional PISA data. The study used the pooled data from all countries participating in PISA 2002, 2003, and 2006 to estimate a model with student fixed effects. The basic idea was that the student fixed-effects absorb any student, school, and country characteristics that are not subject-specific so that they cannot bias the estimation of the instructional time effect. The study shows a small but statistically significant positive effect. Furthermore, the study shows that methodology matters because simple OLS estimates (correlations) were up to three times larger indicating severe selection bias (Hanushek, 2015). Although Lavy’s study has a strong research design, there are also limitations. First, the study did not estimate the effectiveness of instructional time on achievement for different countries but for the pooled international data. Instructional time may be more effective in some countries than in others. Second, the study is limited to secondary school data so that the findings may not be valid for primary school. Thirds, PISA surveyed students about the amount of instructional time and this information may be unreliable.
The main aim of the present study is to estimate the effect of instructional time on student achievement. We build on the work of Lavy (2015) using differences in instructional time across subjects but extent the analyses in three ways. First, we focused on students in primary school instead of secondary school. Second, we conducted analyses country-by-country instead of pooling the international data. This approach allows us to explore international differences in the effectiveness of instructional time. Third, we use teacher-reported data on instructional time, which we consider a more valid measure of instructional time than student data.