Prior to this point, items were calibrated at the point each subject was rolled out, which happened over the course of some years, some subjects before and some after the COVID-19 pandemic.
Cyn hyn, roedd eitemau yn cael eu graddnodi ar adeg cyflwyno pob pwnc, a ddigwyddodd dros gyfnod o rai blynyddoedd, rhai pynciau cyn a rhai ar ôl pandemig COVID-19.