Block Prorating in Sitewise
Block Prorating is the method used in Sitewise to determine which demographics to pull when a study area is created. When calculating the variables for a study area, Sitewise goes to the block level to include all blocks whose centroids fall within the study area shape.
At the block level, only the figures for population and households are published and the rest of the demographics are at the blockgroup level. Since we use blocks to calculate the study area demographics, we need to make sure all demographic values are at that geographic level first. Luckily blockgroups are a collection of blocks. So, for the remaining demographics at the blockgroup level, our team is able to use a factor that is applied to the blocks contained within each blockgroup, to deduce the demographic values for each block.
We use the same method for Canadian data, except using their different census geographies, going down to the Dissemination Block level and using demographics from the containing Dissemination Area.
Block Prorating in Practice
- A block has a population of 1000 people
- It is within a blockgroup that has a population of 5000 people and 1000 homeowners
- 1000 / 5000 = 0.2, so a factor of 0.2 is applied to block population to get homeowners
- 0.2 * 1000 = 200, so the block is given a count of 200 homeowners
Block Proration assumes that the demographics within a blockgroup are evenly spread across its blocks. It provides greater accuracy with block-level counts rather than using a simple proportion of area. This is important consideration because geographical areas without population, such as parks, are not given any demographic values. In return, this makes demographic counts for small study areas more accurate.
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