When working with data, there are a lot of choices to be made. Detailed below are the methods the mySidewalk team leverages to get data into custom boundaries and how we make historical data fit into new boundaries.
There are main 2 reasons the numbers you are seeing in mySidewalk may differ from other sources:
Dynamic calculation of data for your custom boundary
We harmonize historical data to the most up-to-date geographic boundary.
1. Dynamic calculation of data for your custom boundary (Apportion)
You can upload or draw a custom boundary within the mySidewalk Platform. When you do, the mySidewalk Platform can automatically calculate data for your custom boundary.
This works by leveraging 2 things:
The Decennial Census block ratio tables, which provide the count of population, households, and housing units from each census block.
Data available for Census block groups or if no data for block groups, then data available for Census tracts.
When you have a custom boundary, mySidewalk looks up all of the contained Census blocks. It then uses the population, household, or housing unit ratio to apportion the value from the Census block group, or tract, into your custom boundary.
Why use the blocks?
Where people live is not evenly distributed across a Census block group or tract. There may be parks, industrial districts, parking lots, or retail areas. The Census blocks reflect this uneven distribution so that we can more accurately take the values from Census block groups or tracts and calculate them for your custom boundary.
Which - population, households, or housing units - will be used?
Most Decennial and ACS Census data falls into one of those three categories - population, households, or housing units. A household is a group of people living together; a group of people. Whereas housing units describe the physical structure in which people live. The mySidewalk team maintains a metadata ecosystem that keeps track of the best fit ratio type.
Pro-Tip: The accuracy of a single or even a dozen census blocks can suffer from errors. That’s why when you use mySidewalk, you will see a warning if your geography does not overlap at least 40 Census blocks and at least 3 different Census block groups or tracts.
2. We are using the most up-to-date geography.
Census geographic boundaries update at varying intervals and the team at mySidewalks works to make sure we have the most up-to-date set available.
Part of using the most up-to-date boundary is the harmonization of historical data into these new boundaries. This involves taking historical data which was produced for a different boundary and recalculating the historical data so that it fits in the modern boundary, as shown in the diagram below.