Out of curiosity, what period are we talking about here - or, indeed, is it just census data in general down to the present day? Early census data is considerably less reliable than later data, accuracy improving substantially in the middle of the nineteenth century.

I'm not sure exactly what the original poster was referring to. The census data I use is that drawn from the England and Wales censuses from 1841 - 1901 -- these being the ones I can access easily from the comfort of my own home. On the whole I've found it pretty accurate, or at least when all six are viewed together a reasonable consensus generally appears to emerge, except in the case of age which, in some cases, is all over the place and occasionally gives one cause to doubt you are even looking at the same family... There are odd bizarrenesses though and, of course, apparently missing families even when you go through the census for an area page by page and don't rely on the searchable indexes.