It can be the case that despite exhaustive investigation of a census, using all possible search techniques, an individual or family remain elusive. There can be many reasons for this, from a serious transcription error in a finding aid, to deliberate evasion at the time. In this case, an entire household in Streatham in 1841 are all recorded with names being N. K. which was the census enumerators shorthand for 'not known'.
The household is made up of adults, ranging in age from 20 to 45. Given that the census enumerator was told to round ages down to the nearest five in this census, and the names of the individuals were not known, it was likely that these were approximate guesses. None of the household were born in the county in which they were then living, as indicated by the letter 'N' in the final column', although again it was likely that this was a guess.
All the males in the household were shown to be working as agricultural labourers. The date of this census, being 6th June 1841, perhaps has a bearing on the lack of information known about them. They were perhaps only temporarily resident here due to seasonal farm work, and were not known familiarly to neighbours, who could only provide vague details when the census enumerator called.
If you have encountered any problems with identifying your ancestors in a census returns, contact us to see if we can help with your searches.

