An experimenter determined whether children would be assigned to an intervention group or a control group based on the school they attended. This approach may have introduced systematic variation that had nothing to do with the intervention, called , due to non-random assignment.

Which of the following activities would conserve the most energy? installing a dimmer switch on a light using a blender instead of a food processor hanging clothes to dry on a clothesline instead of ironing them opening a window instead of turning on the air conditioning setting your computer's screen saver to turn on after less time has elapsed