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9 used anonymous cell phone No matter where you are in the world, a mask is going to give you the same protection. Unfortunately, many other NPIs aren't so universal. Their effectiveness depends a lot not only on when they're used, but on where they're used too. Lockdowns are a great example. The evidence is clear that they reduce transmission, and that stricter lockdowns reduce transmission more. But they're not equally effective everywhere, because not everyone is able to comply by staying in one place. The difference is actually quantifiable. One ingenious study9 used anonymous cell phone data from throughout the United States to gauge the extent to which people living in different neighborhoods stayed at home. (Your cell phone checks in periodically with a service that establishes its location.) Between January and March 2020, people who live in America's wealthiest neighborhoods were the most mobile--that is, they spent the most time out of their homes--and people in the lowest-income neighborhoods were the least mobile. But in March, as lockdowns kicked in across the country, the situation swung upsidedown. People in wealthy neighborhoods became the least mobile, and people in the poorest ones were the most mobile. The reason: They were much more likely to have jobs outside the home, and much less able to use delivery services for groceries. A similar shift was driven by population density. Before lockdowns, the densest communities had the highest transmission rates. After lockdowns, they had the lowest transmission rates, and the less-crowded areas had not dropped nearly as much. That makes sense, of course, because when people aren't living and working in close quarters to begin with, telling them to stay home will naturally have less of an impact on transmission.