when using the census you need to consider the implications of changes over time, for example, the questions change and this has implications for being able to compare results for one year to another.
When using the nineteenth century census, this means you need to be aware, for example, that the same term for an occupation might mean something different decades later (implications of changes to occupations for using the census). You need to be aware that because of the changes to the questions asked about birthplaces, different censuses won't provide exactly the same kind of information - you mustn't miss subtle changes to the meaning of fields and considering what the implications of these might be. In general, older censuses provide less information and less detail than later censuses, though the trend is not entirely one way - in 1931 the scope of the census was reduced because of the effects of economic depression.