Question:
I got a good question today! A student noticed that “the stats Canada definition for Dissemination Areas is supposed to be between 400 and 700 people, but some of the DAs have populations over 1,000 and upwards of 6,000 (DA: 12090839) for example.”
I told the student that the Census Dictionary definition for DAs says that "Dissemination areas are uniform in terms of population size, which is usually targeted from 400 to 700 persons to avoid data suppression. DAs with lower population counts (including zero population) may result in order to respect the boundaries of census subdivisions and census tracts. DAs with higher population counts may also result.”
I looked at the map of the DA he mentioned, and it did seem to me like there might not be many options to subdivide it further because of how the streets kinda curl into a giant neighbourhood. But I promised I would check. Is it possible for a DA to have a population of over 6000? Is the census dictionary definition the best explanation we have on that topic?
Answer:
Here is the general reasoning I was able to find behind the situation here. It goes a little bit more in-depth than was is provided in the Census Dictionary, but basically covers the same ground:
“The introduction of the national block program in 2001 allowed for the de-coupling of collection and dissemination geographies. Prior to 2001 the enumeration area (EA) was the smallest unit for the collection and dissemination of census data. However, the EA was designed to be small enough for census collection and therefore was not always large enough for census dissemination – resulting in some data suppression for small area geography. The dissemination area (DA) was introduced in 2001 – new DAs (replacing EAs) were delineated using an automated delineation tool in all census tract areas of the country; however, outside of census tracted areas the DAs continued to follow the 1996 EA limits (collection and dissemination geographies were not de-coupled in these areas). The DAs in the non-tracted parts of the country were completed re-delineated using an automated delineation tool for the 2006 census – at this time the DAs in tracted parts of the country were split as a result of high level boundary changes or where growth dictated.
Current situation:
DAs are designed to respect both census tract (CT) and census subdivision (CSD) boundaries with an optimal population size of between 400 and 700. However, in densely populated portions of the country (e.g. areas with high concentrations of high rise apartments) DAs can be quite large (over 1000 dwellings) but may be composed of just one or two dissemination blocks, thus making it impossible to reduce the population of the DA. Conversely, in rural parts of the country with very low population density and with the constraint that DAs must respect CSDs (some CSDs have less than 100 people or are made up of two or more discontiguous polygons with very low population counts) DAs can be quite small (50 – 100 dwellings), thus making it impossible to create a larger DA that would contain a population within the ideal 400-to-700 range.”
