There was a study conducted to estimate the occurrence of COVID-19 among workers in a workplace that piloted the use of robot cleaners each night to see if they would be more effective at preventing COVID-19 than human cleaners. A total of 600 workers were enrolled in the study. All were tested for COVID-19 at the beginning of the study and all were antibody negative (therefore, assume all are at risk). All study participants were tested again four months later (assume no one was lost to follow-up and all participants completed the study).

A total of 260 workers worked shifts following cleaning by robots, and the study found that 80 of these workers tested antibody positive for COVID-19 at the four-month study endpoint. Of the 340 workers who worked shifts following cleaning by humans, 130 eventually tested antibody positive.

What type of study design was used to evaluate this intervention? (0.5 points). How do you know? (0.5 points).