Non-sampling errors are not attributed to chance and are a consequence of certain factors which are within human control. In other words, they are due to certain causes which can be traced and may arise at any stage of the enquiry, viz., planning and execution of the survey and collection, processing and analysis of the data.
Non-sampling errors are thus present both in census surveys as well as sample surveys. Obviously, non-sampling errors will be of large magnitude in a census survey than in a sample survey because they increase with the increase in the number of units to be examined and enumerated.