Dictionary, Census of Population, 2021
Figure 1.10
Historical comparability rule

Figure 1.10 is a graphical representation of the historical comparability rule (rule 5^{Note 1}) for delineating a census metropolitan area (CMA). Two fictional groupings of 12 census subdivisions (CSDs), shown as polygons labelled "A" through "L," represent the groups of CSDs that could form a CMA. The groupings are layered one on top of the other. Both are identical except the grouping on top is labelled "Current census," while the grouping underneath is labelled "Previous census" and CSD L does not comprise two parts.

All of CSD A and a small portion of the adjacent CSDs B and J are shaded to represent a population centre. This population centre also makes up the core of the CMA. CSD D has a small shaded polygon within it that identifies a population centre that is not part of the core and is therefore referred to as the fringe.

An arrow extends from CSD I in the previous census layer to CSD I in the current census layer. This shows that CSD I is included in the current version of the CMA because it was previously included in the CMA.

A thick black line outlines the perimeter of CSDs A, B, C, D, E, F, G, H and I and excludes CSD K, leaving a hole, to identify the new boundary of the CMA.

Two legends appear below the figure. The legend on the left identifies the symbols used to represent the boundaries of the CMA, CSDs, population centres, core and fringe. The legend on the right shows which CSDs are included in the CMA according to which rule. For this figure, CSDs A and B are included under rule 1 (delineation core rule); CSD C is included under rule 2 (forward commuting flow rule); CSD D is included under rule 3 (reverse commuting flow rule); CSDs E, F, G and H are included under rule 4 (spatial contiguity rule); and CSD I is included under rule 5 (historical comparability rule).

Source: Statistics Canada, 2021 Census of Population.