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%). Ten seedlings were randomized to each level of control. The resulting sample means for stem volume were 58, 74, and 120 cubic centimeters (cm3) with sp = 25 cm3. The researcher hypothesized that the average growth at 50% control would be less than the averages of the 0% and 100% levels.
(a) What are the coefficients for testing this contrast?
(b) Perform the test. Do the data provide evidence to support this hypothesis?