As a result changes in trend need to be interpreted with a degree of caution, as changes could be attributed to sampling effects as well as real effects.
O ganlyniad, mae angen i newidiadau mewn tueddiadau gael eu dehongli’n ofalus, oherwydd gellid priodoli’r newidiadau i effeithiau samplu yn ogystal ag effeithiau gwirioneddol.