Pooling four years allowed for data to be broken down into four categories, improving granularity, however conclusions were still limited in terms of precision.
Wrth gyfuno pedair blynedd roedd hi’n bosibl dadansoddi’r data mewn pedwar categori, gan wella gronynogrwydd; fodd bynnag, roedd y casgliadau’n dal yn gyfyngedig o ran manwl gywirdeb.