Several international large-scale assessments in education take place every 3 to 5 years. PIRLS (the
Progress in International Reading Literacy Study) takes place every 5 years. If data from only one
moment are considered, it is difficult to explain differences in achievement between countries. But
we can also consider data from two (or more) years and focus on changes over time within each
of the participating countries. Following countries over a certain period in such a longitudinal approach
facilitates causal inferences on the effects of characteristics of educational systems.
In this study, we investigate the effects of instructional time for language and reading and of amount
of professional development of teachers on reading comprehension in Grade 4. We also study the
effect of changes in the average age of students. By accounting for the effects of age and schooling
we come to a slightly different ranking of countries in PIRLS. We use a difference-in-difference approach
with correlation matrices. One of our conclusions is that professional development of teachers
has an effect on the achievement level of students.