Boston PD only required 100 out of 2,000 cops to wear body cams. Then argued they shouldn't be required to have any cops wear body cams because it didn't change the number of complaints despite the fact the 100 cops they choose were ones who had never gotten complaints

https://www.boston.com/news/local-news/2018/01/11/bostons-early-results-show-small-benefit-from-body-cameras