A quadtree does the same thing for two-dimensional space. It takes a rectangular region and divides it into four equal quadrants: northwest, northeast, southwest, southeast. If a quadrant has too many points in it, it subdivides again and again. Each subdivision creates smaller and smaller cells where points are densely packed.
The report pointed out how birth rates in the U.S. have been below the minimum replacement rate since 2008, meaning that the bulk of population growth since then has been the result of immigration. This has proved especially true for the country’s labor force. Nearly 80% of immigrants are of working age, according to the Census, and they account for 19% of the workforce, around 33 million people.
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