There were a number of important differences in the way information was collected and processed in the 2001Census, compared to previous censuses. Concerns over the level of underenumeration in the 1991 Census (link to coverage unit), and particularly a problem of differential underenumeration (between areas and types of people), led to a major rethink over the best way to conduct the 2001 census. A major concern was how to maximise coverage and minimise the bias introduced by differential undercount.