the ons coding system is a system used in the united kingdom for dealing with census and other statistical data. it is used by the office for national statistics as a way to label each area of england and wales. it is also used for parts of northern ireland. the system uses a code for the larger census areas, counties and districts. it then breaks those areas down into smaller areas within the larger areas. each of the smaller areas get a code that is based on the larger area. areas are divided in this way until they get to the smallest size used for census data. an area must have at least 40 households, but they try to not divide an area into a size smaller than 100 households.