Using the VisionLTC platform, your can define custom geometric areas to use as the basis of a market area calculation. This article is meant to provide a brief overview of how data retrieval for those defined market areas occurs.

VisionLTC uses the block centroid data retrieval. The process works as follows:

Data are stored at the census block group level - the lowest geographic level for which most census data and Nielsen estimates are reported. However, Nielsen estimates for population, households, and housing units are carried at the census block level - the smallest level of census geography.

When an area such as a five-mile radius is defined, the system identifies which census blocks are included in the area. Inclusion is determined by latitude/longitude points within each block, referred to as centroids. If a block's centroid is included in the radius, the block is allocated to the area.

The list of blocks allocated to the area determines the inclusion factor for block groups. If all blocks in a block group are included in the area, 100% of the block group's data is allocated to the area. If only some of the block group's blocks are in the area, the percent inclusion is based on the population/household/housing-unit estimates for census blocks. For example, consider a block group with 30 blocks and a total estimated population of 1,500. If 10 of those 30 blocks are within the geometric area, and those 10 blocks have a combined estimated population of 1,000, that is an inclusion factor of 66% (1,000/1,500). The system would allocated 66% of this block group's data to the geometric area. All block group data would be allocated with the 66% factor. Note that while only 33% of the block group's data blocks are in the area, the block data indicate a 66% share of the block group's population, and thus a 66% allocation of the block group's data to geometric area. Depending on the metric being calculated, population, households, or housing-unit counts are used automatically.