FORESTRY
AGRICULTURAL STATISTICS AND ECONOMICS
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Question
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The power takeoff driveline on tractors used in agriculture is a potentially serious hazard to operators of farm equipment. The driveline is covered by a shield in new tractors, but for a variety of reasons, the shield is often missing on older tractors. Two types of shields are the bolt-on and the flip-up. It was believed that the bolt-on shield was perceived as a nuisance by the operators and deliberately removed, but the flip-up shield is easily lifted for inspection and maintenance and may be left in place. In a study initiated by the U.S. National Safety Council, random samples of older tractors with both types of shields were taken to see what proportion of shields were removed. Of 183 tractors designed to have bolt-on shields, 35 had been removed. Of the 136 tractors with flip-up shields, 15 were removed. We wish to perform a test of H0:pb = pf versus Ha:pb > pf, where pb and pf are the proportions of all tractors with the bolt-on and flip-up shields removed, respectively. Which of the following is not a condition for performing the significance test?
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The data come from two independent samples.
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Both samples were chosen at random.
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Both populations are Normally distributed.
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The counts of successes and failures are large enough to use Normal calculations.
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Both populations are at least 10 times the corresponding samples sizes.
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Explanation:
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