Refer to the International Journal of Foresting Engineering (Vol. 19, 2008) investigation of the muscle activity patterns in the neck and upper extremities among forestry vehicle operators, Exercise 7.38. Recall that two types of harvesting vehicles — Timber jack and Valmet — were compared using an independent-samples design. How many Timber jack and Valmet harvester operators should participate in the study in order to estimate (μ_T - μ_v), the true mean difference in muscle rest (seconds per minute) for the two harvesting vehicles, to within 1.5 seconds/minute with 90% confidence? Assume equal sample sizes.

Data from Exercise 7.38

Research in the International Journal of Foresting Engineering (Vol. 19, 2008) investigated the muscle activity patterns in the neck and upper extremities among forestry vehicle operators. Two types of harvesting vehicles — Timber jack and Valmet — were compared since they differed dramatically in design of the control levers. Independent random samples of 7 Timber jack and 6 Valmet harvester operators participated in the study. Muscle rest (seconds per minute) in the right trapezius (a neck muscle) was determined for each operator. The 7 Timber jack operators had a mean muscle rest of 10.35 seconds/minute, while the 6 Valmet operators had a mean of 3.30 seconds/minute.