Rotary oil rigs. An economist wants to compare the average monthly number of rotary oil rigs running in three states: California, Utah, and Alaska. In order to account for month-to-month variation, three months were randomly selected over a two-year period and the number of oil rigs running in each state in each month was obtained from data provided from World Oil LO4

(Jan. 2002) magazine. The data, shown in the accompanying table, were analyzed by means of a randomized block design. The MINITAB printout is provided on the next page.

Month California Utah Alaska 1 27 17 11 2 34 20 14 3 36 15 14

a. Why is a randomized block design preferred over a completely randomized design for comparing the mean number of oil rigs running monthly in California, Utah, and Alaska?

b. Identify the treatments for the experiment.
Supplementary Exercises 10.110–10.141 603 MINITAB Output for Exercise 10.120

c. Identify the blocks for the experiment.

d. State the null hypothesis for the ANOVA F-test.

e. Locate the test statistic and p-value on the MINITAB printout above. Interpret the results.

f. A Tukey multiple comparison of means (at a = .05)
is summarized in the SPSS printout below. Which state(s) have the significantly largest mean number of oil rigs running monthly?

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Two-way ANOVA: NumRigs versus State, Month/Year Source DF SS MS F P State 2 617.556 308.778 38.07 0.002 Month/Year 2 30.889 Error 4 32.444 15.444 8.111 1.90 0.262 Total 8 680.889 S = 2.848 R-Sq 95.23% R-Sq (adj) = 90.47 % State Mean AL 13.0000 CAL 32.3333 UT 17.3333 Individual 95% CIs For Mean Based on Pooled StDev (---------) (---------) ------ (---------) ---- 16.0 -+ 24.0 32.0 40.0