12.30 Growth of Douglas fir seedlings. An experiment was conducted to compare the growth of Douglas fir seedlings under three different levels of vegetation control (0%, 50%, and 100%). Sixteen seedlings were randomized to each level of control. The resulting sample means for stem volume were 58, 73, and 105 cubic centimeters (cm3), respectively, with sp = 17 cm3. The researcher hypothesized that the average growth at 50% control would be less than the average of the 0% and 100% levels.

  1. (a) What are the coefficients for testing this contrast?

  2. (b) Perform the test and report the test statistic, degrees of freedom, and P-value. Do the data provide evidence to support this hypothesis?