Refer to the Wind Engineering (Jan. 2004) study of two types of timber radiata pine and hoop pine—used in high-efficiency small wind turbine blades, Exercise 10.16. Data on stress (y) and the natural logarithm of number of blade cycles (x) for each timber type were analyzed using simple linear regression. The results are reproduced here, with additional information on the coefficient of determination. Interpret the value of r² for each type of timber.

Radiata Pine: ŷ = 97.37 - 2.50x,  r² = .84

Hoop Pine:    ŷ = 122.03 - 2.36x, r² = .90

Data from Exercise 10.16

Mechanical engineers at the University of Newcastle (Australia) investigated the use of timber in high-efficiency small wind turbine blades (Wind Engineering, Jan. 2004). The strengths of two types of timber—radiata pine and hoop pine—were compared. Twenty specimens (called “coupons”) of each timber blade were fatigue tested by measuring the stress (in MPa) on the blade after various numbers of blade cycles.