# How are the radius report estimates calculated?

First, we subtract the 2022 US Census Bureau’s water areas from the 2022 block groups. Then a geographic information system (GIS) analysis calculates the project's radius(es). The GIS analysis then identifies the 2022 US Census Bureau block groups minus the water areas that intersect the project's radiuses. Then, the GIS analysis calculates the percent of each block group within each radius distance (overlap). Next, the overlap percent is multiplied by the demographics for each block group. Finally, the radius demographic estimate equals the sum of the overlap multiplied by the demographics for all block groups that intersect a radius.

The benefits of the methodology are that it allows for:

1. the use of the most current data for small area geographies from the US Census Bureau;
2. the estimation of demographics for radius distances using dissimilar shaped Census block groups; and
3. data comparability (because estimates for small radiuses and large radiuses use the same methodology, geographies, and data sets).
4. improved estimates along coastlines and large water bodies by removing water areas.

This methodology assumes that the population is equally distributed throughout a block group. This assumption can result in unlikely estimates for small radiuses (i.e. 1 mile) in rural areas with low population densities and thus, large geographic area block groups.

## Median Calculations

To estimate median values, use a simple method explained in a book called "Beginning Statistics: An Introduction for Social Scientists" by Diamond I and Jefferies J. This formula is applied to the calculations created with the overlaps above. Here's how it works:

1. We pull grouped data for your area of interest and order the groups from smallest to largest. This helps us see which group the middle value, or median, is in.
2. We figure out exactly where the middle value sits within its group. We look at how far into the group it is, as a percentage, starting from the bottom end.
3. Then, we check how big the gap is between the smallest and largest numbers in that group.
4. We use the percentage to find a part of that gap. This percentage tells us how much to add to the smallest number in the group to get the median.