In a study of the effect of playing video games on sleep quality, researchers randomly assigned some participants to play an up-tempo video game for either one or three hours before bed. Other participants had no engagement with electronics in the time before bed. Then, the researchers assessed all the participants' sleep quality (e. g., number of awakenings, percentage of time in deep sleep) during an overnight sleep study. In this experiment, the time spent playing video games (e. g., one or three hours) is the .