They do not attempt to predict the impact of government policies, changing economic circumstances, or other factors (like the coronavirus pandemic) on future population.
Nid ydynt yn ceisio rhagweld effaith polisïau’r llywodraeth, newid mewn amgylchiadau economaidd na ffactorau eraill (fel pandemig y coronafeirws) ar boblogaeth y dyfodol.