通过除去移民因素，合并人口较少的县，他们把美国的3141个县分为了2068个“县级单位”；再调查每个县的死亡率。
They looked at death rates by county, having corrected for migration and merged sparsely populated ones so that America's 3, 141 counties became 2, 068 “county units”.
通过除去移民因素，合并人口较少的县，他们把美国的3141个县分为了2068个“县级单位”；再调查每个县的死亡率。
They looked at death rates by county, having corrected for migration and merged sparsely populated ones so that America's 3, 141 counties became 2, 068 “county units”.
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