The duration of pre-primary education is measured in number of grades (years). These estimates come from official country reports to UNESCO UIS annual education surveys. The definition of 'pre-primary' corresponds to the International Standard Classification of Education (ISCED). The reference years reflect the school year for which the data are presented. In some countries the school year spans two calendar years (for example, from September 2010 to June 2011); in these cases the reference year refers to the year in which the school year ended (2011 in the example).
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