When stakeholders ask you questions, we want you to know and trust that the data you have from the mySidewalk Platform is accurate.
Data can be complicated and part of our job is to simplify it, but we want you to have access to both data sources and our processing methodology so you can understand where your data comes from.
When working with data, there's a lot of choices to be made. Scope, scale, and time play a big role in how mySidewalk will collect and analyze your data. To get a better idea about how we reach our calculations and why they may differ from other sources, we have listed our methodology on how we group geography together along with what level of accuracy we work with.
You may be here because:
- You want to know more about the data before diving into it.
- You have numbers from your organization that aren’t the same as the number mySidewalk is showing from your custom geography.
- You love #allthingsdata as much as we do!
There are 3 reasons the numbers you are seeing in mySidewalk may differ from the ones your organization is utilizing:
- We use weighted block apportionment.
- We use census blocks, the smallest geography available, to get the most granular result.
- We are using the most up to date geography.
1. We use weighted block apportionment.
When creating a custom geography in mySidewalk, the boundaries you create will be drawn on top of census blocks that contain data. This shape might fit exactly to a set of census blocks. If it does, you numbers will be directly taken from all the census blocks contained in your study area.
If your shape covers only 40% of the underlying census block, you might think that the platform will give back 40% of the population in that tract. This is not the case. When you create a new shape, the platform will imagine a dot in the center of each of the census blocks. If the dot lands inside your new shape, your shape will display 100% of the data in that census block, if not, we don’t add any of the data to your new shape.
Why would I want to ignore data if my shape is overlapping with some portion of the census block?
Geography is not evenly distributed and neither is the data in a census block. There may be parks, industrial districts, or retail in a census block with neighborhoods. We don’t know exactly where these elements are located in the census block when we’re making calculations on any given shape, but using weighted block apportionment will let us feel comfortable that the census blocks within your shape can be incorporated into any calculations.
If you select a small part of a census block that is 80% commercial, you wouldn’t want to capture the data from the 20% of the shapefile that is part of a neighborhood. This would create an inaccurate view of your selected area. Ignoring the shape all together is more accurate.
Pro-Tip: Census blocks are small areas with a minimum size of .69 acres. If you miss one or two, it won’t make a huge difference in total numbers or the accuracy of the data portrayed.
2. We use census blocks, the smallest geography available, to get the most granular result.
In order to get the most accurate numbers for your custom shape, we add together each census block inside your shape. Census blocks are the smallest geography that the Census Bureau uses when conducting their surveys. Using a small geography gives us the flexibility to make high confidence calculations about your geography, no matter how unconventional the shape.
Pro-Tip: Even though census blocks have the ability to give more fine grained measurements about our shapes, this is only the case when they are used in large groups. 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 block points. If you’re just wanting to show the shape of your city’s park or tax district, this may not be a concern for you, but be wary if you plan to make measurements and calculations on these small geographies.
3. We are using the most up to date geography.
Boundaries are often updated or refined to be more accurate. mySidewalk wants to make sure that when you are comparing data from the past, we are keeping you in the same context. You might have some data about average income from your city in the 1980s, but since then the city has grown and now includes people who live on the other side of the river that previously was not part of your city. When you look at the average income for your city in the 1980s, we now account for that new expansion and include it in the calculation, even if it’s true that this tract of land wasn’t part of the city back then.