It has often been lamented that girls are more likely not to attend or to drop-out (for example, because of gender preference by the parents, insecurity, pregnancy, amongst many other postulated factors) or that their achievement even if they stay in school is worse than boys. For the former, UNESCO introduced the Gender Parity Index (GPI), which measures the relative participation of boys and girls at each schooling level, reflecting the enthusiasm and motivation both of girls relative to boys and of their parents' priorities for their daughters and sons. The purpose of this research note is to explain how the analysis of girls' educational performance would also benefit from the comparison of the achievement of girls and boys, with GPI-A measuring the ratio of girls' achievement to boys' achievement.