In Chile the question about the certification of teachers was adapted as "authorised or enabled by the Ministry of Education". The timing of the PISA data collection can have an impact on principals' responses to this question. For example, if most teachers in a country or economy had participated in professional development programmes during summer holidays and the PISA data collection was conducted before the summer break in this country/economy, the reported proportion would be an underestimate of the reality. The index of equity in resource allocation (staff) is the percentage of the variation on the index of shortage of educational staff explained by the school PISA index of economic, social and cultural status of the school multiplied by a negative or positive sign, depending on the sign of the relationship.