Dr. Jones is interested in studying how white noise can influence participants' sleep quality. A sample of 100 participants is selected. Fifty of the participants are randomly assigned to the white noise condition where they are instructed to use a white noise machine in their bedrooms during sleep. For the other 50 participants, they are to sleep in their normal noise conditions. Each week for two months, Dr. Jones records the self-reported fatigue levels of the participants in each of the conditions. In Dr. Jones' Study, the color of the walls in the participants' bedrooms is variable.