In 2009, Scott Ross and Robert Horner conducted a study illustrating the multiple-baseline design across participants by evaluating a school-wide bullying prevention program. They observed the aggressive behaviors of specific problem students across three different schools to establish a baseline. They then implemented the prevention program at the first school, waited two weeks to implement it at the second school, and waited another two weeks for the third school. Because the aggressive behaviors of the students dropped shortly after the program was implemented at their respective schools, the staggered introduction made it highly unlikely that the changes were due to external coincidences, demonstrating the internal validity of the design.