In planning for a new forest road to be used for tree harvesting, planners must select the location that will minimize tractor skidding distance. In the Journal of Forest Engineering (July 1999), researchers wanted to estimate the true mean skidding distance along a new road in a European forest. The skidding distances (in meters) were measured at 20 randomly selected road sites. These values are given in the accompanying table and saved in the SKIDDING file.

a. Estimate, with a 95% confidence interval, the true mean skidding distance of the road.
b. Give a practical interpretation of the interval you found in part a.
c. What conditions are required for the inference you made in part b to be valid? Are these conditions reasonably satisfied?
d. A logger working on the road claims that the mean skidding distance is at least 425 meters. Do you agree?

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488 350 457 199 285 409 435 574 439 546 385 295 184 261 273 400 311 312 141 425