The nineteenth century census materials likely underestimate the total population. Results may be unreliable where people are highly mobile or unwilling to be enumerated, or where enumerators were required to travel long distances. Groups of people such as the poor, transient workers, fluid populations and ethnic minorities are often under-enumerated compared to people in the middle class or the social and economic elite. In the religion field results should also be treated with caution. For instance, if a Native person was married to a non-Native, he or she was usually assigned the religion of the spouse.
Problems are also apparent with the concept of occupational status.
In the 1881 and the 1891 Canada Censuses, for example, enumerators often left the occupation column blank, particularly in the case of women, even though they may well have been working as operators of farms, fishers, or other occupations. In addition, it is probable that the enumerators did not have an understanding of Native economies and could not relate Indian labour to occupation categories clearly descended from the European experience. Enumerators also had difficulty rendering Native people's names in English and for this reason individuals may be referred to using different spelling in different censuses. Names present other problems for the researcher. A Native person may be identified by a Native name in the 1881 Canada census, by a Christian name in the 1891 Canada Census, and by taking their father's Christian name as a surname in the 1901 Canada census.