Schooling as a whole is an essential element in increasing students’ performance in PISA, and each additional year of school completed might benefit students in their overall academic performance. In many countries the 15-year-olds in the PISA sample are distributed across the seventh to twelfth grades, and although the students are mainly enrolled in one or two modal grades, 15-year-olds can receive a varied number of years of schooling. Although a positive relationship between grade level and performance is reported by the OECD, it remains unknown to what extent this effect is associated with other student characteristics like their age (which might vary by 11 months), SES, grade repetition and so on. While the select bias that exists between students enrolled in tenth grade or higher and those in lower grades remains out of control the true value of an additional year of schooling is unknown.
Hypothetically if the effect of one additional year reflects the effectiveness of schooling, the size of this effect is expected to be linked to overall national results in PISA. More specifically, the countries which show higher performance in PISA are expected to have  larger   a rapid rise of overall scores in PISA from wave to wave might have experienced a larger effect of one additional year of schooling. 
The primary aim of this study is to examine the impact of one additional year of schooling on achievements in PISA addressing the selection bias problem. Secondly this study examines the relationship between this effect and country performance in PISA.

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