If the makeup of the countries changes across the years, how can one compare countries to the PIRLS scale average?
• PIRLS scores are reported on a scale from 01,000 with the scale average fixed at 500 and a standard deviation of 100. The PIRLS scale average was set in 2001 and reflects the combined proficiency distribution of all students in all jurisdictions participating in 2001. To allow comparisons between 2001 and 2006, scores of students in jurisdictions that participated in both 2001 and 2006 (29 jurisdictions) were used to scale the 2006 results. The 2006 scores were linked to the 2001 scale using items common to both assessments. Once scores from the 2006 assessment were scaled to the 2001 scale, scores of students in jurisdictions that participated in 2006 but not in 2001, were placed on the PIRLS scale.
