Non-Response Bias Analyses were carried out to estimate the amount of bias within Wales’ sample, and the sample of other countries that did not meet PISA standards.
Cynhaliwyd Dadansoddiadau o Duedd Diffyg Ymateb i amcangyfrif faint o duedd sydd yn sampl Cymru, a'r sampl o wledydd eraill nad oeddent yn bodloni safonau PISA.