Most data-driven discussions about race focus either on the national level (which masks local trends) or are centered on areas of conflict (such as Ferguson, Mo.) USA TODAY wanted to give people the tools that would allow them to explore how race end ethnicity have changed over time -- where they live and where they go to school. But how do you measure diversity when such trends wax and wane over time? Is a community that changed from nearly all-white to all-black as diverse as an area that received a high level of immigrants? Why do some communities barely notice big changes over time, while others become a nexus of violence? And how does the change in my community compare to anyone else's? To do that USA TODAY needed a tool to level the playing field, a way to show 100 years of change both locally and nationally, on the same scale. The series, based on the USA TODAY Diversity Index, is explanatory data at its best: quantifying incremental change that everyone sees anecdotally.