Remember, there's a natural tendency for human beings to want to confirm their hypothesis. If she thinks that bus A is faster, maybe she'll want to pick the days where she'll get data to confirm her hypothesis. It's really important that she randomized the 50 workdays. What I can imagine she did is maybe she wrote each of the workdays, the dates, on a piece of paper. She would have 50 pieces of paper, and then she turned them all upside down, or maybe she closed her eyes, and then she moved them all over her table. Then with her eyes closed, she randomly moved them to either the left or the right of the table. If they move to the left of the table, then those are the days she'll take bus A.