Joseph DiFranza of the University of Massachusetts Medical School in the United States, monitored nearly 700 students, who were aged 12 and 13 at the start of the study.
Džozefs Difransa no Masačūsetsas universitātes medicīnas fakultātes (ASV) divarpus gadu laikā novēroja aptuveni 700 skolēnus, kas pētījuma sākumā bija 12 un 13 gadus veci.