To fully understand census data as applied to our ancestor family, it is important to know what the definitions of certain elements meant. The census instructions defined the following:
Private family - Any person or group of persons included in a separate return as being in separate occupation of any premises or part of premises is treated as a separate family for census purposes; lodgers were so treated only when returned as boarding separately and not otherwise. Private families comprise all such families except for those enumerated in institutions or business establishments or boarding houses in which the number of resident trade assistants or resident boarders exceeds the number of members of the employer's or householder's family (including private domestic servants).
There was often a misunderstanding between the term boarder and lodger.
Rooms - For census purposes the rooms enumerated are the usual living rooms, including bedrooms and kitchens but excluding sculleries, landings, lobbies, closets, bathrooms or any warehouse, office or shop rooms.
This definition is consistent with the meaning as shown in earlier census returns where they were defined as habitable rooms.
Structurally separate dwellings - A structurally separate dwelling has been defined for the census as any room or set of rooms, intended or used for habitation, having separate access either to the street or to a common landing or staircase. Thus, each flat in a block of flats is a separate unit; a private house which has not been structurally subdivided is similarly a single unit whether occupied by one family or by several families. But where a private house has been subdivided into maisonettes or portions, each having its front door opening on to the street or on to a common landing or staircase to which visitors have access, then each such portion is treated as a separate unit. Where an undivided private house is used partly for business or professional purposes, it is treated as occupied by a private family unless the portion used for non-domestic purposes consists of at least three rooms and is more than one quarter of the whole.
Overcrowding - The 1921 definition of overcrowding was where there were more than two persons per room, although it should be noted that the 1921 census data give rooms per person and as such can’t be used as evidence of ‘overcrowding’ as defined.
Many of the questions asked in 1921 were far from novel. For the first time, the census required householders to state the sex of persons (rather than having a sex-specific age column) as well as to give ages in years as well as months. Questions as to name, relationship to head, marital status, and birthplace went almost unchanged