A well-known Canadian forestry company carries out a major management comparison: after a clear-cut, half of the 200 plots are planted with new trees, the other half the plots are allowed to...

A well-known Canadian forestry company carries out a major management comparison: after a clear-cut, half of the 200 plots are planted with new trees, the other half the plots are allowed to regenerate naturally. After 20 years the basal area (area covered by trees at breast height) in each plot is measured and compared between treatments. A t-test says that the difference of 7.3 m2/ha is not significant (P > 0.05). The two treatments have an average variance (!) of 321.44. With which level of conviction (= power) can we say that the treatments are the same?