Randomized controlled trials are the "gold standard" for clinical research because they are able to measure cause and effect. In May 2020, the CDC published a comprehensive meta-analysis of all randomized controlled trials that studied the effects of masks on the transmission of the flu in non-healthcare settings. These are public places outside of hospitals where most people wear low-quality masks and are not properly trained on how to fit, use, and dispose of them. While noting that "proper use of face masks is essential because improper use might increase the risk for transmission," the paper examined 10 studies and "found no significant reduction in influenza transmission with the use of face masks." Covid-19 is much more transmissible than the flu, which begs the question of how masks in non-healthcare settings can reduce the spread of Covid-19 when they don't reduce the spread of the flu.