1. 人口：

https://www.census.gov/quickfacts/fact/table/US/PST045219

2.义工：

 https://www.cnn.com/2018/07/19/us/volunteering-statistics-cfc/index.html

3. 美国军队的族裔数据：

https://www.pewresearch.org/fact-tank/2019/09/10/the-changing-profile-of-the-u-s-military/