According to the paper, the random selection was done on participants who expressed interest.
> Approximately half of the employees (503) were interested, particularly those who had less education and tenure, their own rooms, and faced longer commutes. Of these, 249 were qualified to take part in the experiment by virtue of having at least six months’ tenure, broadband access, and a private room at home in which they could work. After a lottery draw, those em- ployees with even-numbered birthdays were selected to work from home, and those with odd-numbered birthdates stayed in the office to act as the control group.
> Approximately half of the employees (503) were interested, particularly those who had less education and tenure, their own rooms, and faced longer commutes. Of these, 249 were qualified to take part in the experiment by virtue of having at least six months’ tenure, broadband access, and a private room at home in which they could work. After a lottery draw, those em- ployees with even-numbered birthdays were selected to work from home, and those with odd-numbered birthdates stayed in the office to act as the control group.