Suppose that we wanted to compare three interventions for preterm infants, with
regard to effects on the infant's heart rates: (1) nonnutritive sucking, (2) nonnutritive
sucking plus rocking, or (3) rocking alone. The 12 infants participating in the study are
randomly assigned to six different orderings of the three treatments. Heart rate
measurements are taken on all 12 infants following each condition.