The use of sampling weights is necessary for computing statistically sound, nationally representative estimates. Adjusted survey weights adjust for the probabilities of selection for individual schools and students, for school or student nonresponse, and for errors in estimating the size of the school or the number of 15-year-olds in the school at the time of sampling. Survey weighting for all education systems participating in PISA 2015 was coordinated by Westat, as part of the international PISA consortium.
The school base weight was defined as the reciprocal of the school's probability of selection multiplied by the number of eligible students in the school. (For replacement schools, the school base weight was set equal to the original school it replaced.) The student base weight was given as the reciprocal of the probability of selection for each selected student from within a school.
The product of these base weights was then adjusted for school and student nonresponse. The school nonresponse adjustment was done individually for each education system by cross-classifying the explicit and implicit stratification variables defined as part of the sample design. Usually about 10 to 15 such cells were formed per education system.
The student nonresponse adjustment was done within cells based first on their school nonresponse cell and their explicit stratum; within that, grade and gender were used when possible. All PISA analyses were conducted using these adjusted sampling weights. For more information on the nonresponse adjustments, see OECD's PISA 2015 Technical Report (forthcoming).