In a census survey , all units from the population of interest are analyzed. A related concept is the sample survey, in which only a subset of the population is taken. The main advantage of the census survey (as compared to the sample survey ) is that the whole population is involved and, therefore, there are no random errors or systematic errors caused by the sampling itself. The main shortcoming is the difficulty of practical realization if the population is large, e.g. the population of a whole country or a large city. These practical difficulties may introduce undercoverage and resulting bias, since the excluded units, being more difficult to reach, are likely to differ systematically from the included units. For this reason, sample surveys where considerable effort can be focused on achieving complete response may perform more accurately than a census of a large population.