答案和详解如下： 1、For a stock, which of the following is NOT a random variable? A) Its most recent closing price. B) Its P/E ratio. C) Its current ratio. D) Its stock symbol. The correct answer was D) A random variable must be a number. Sometimes there is an obvious method for assigning a number, such as when the random variable is a number itself, like a P/E ratio. A stock symbol of a randomly selected stock could have a number assigned to it like the number of letters in the symbol. The symbol itself cannot be a random variable. 2、In any given year, the chance of a good year is 40 percent, an average year is 35 percent, and the chance of a bad year is 25 percent. What is the probability of having two good years in a row? A) 16.00%. B) 10.00%. C) 8.75%. D) 6.25%. The correct answer was A) The joint probability of independent events is obtained by multiplying the probabilities of the individual events together: (0.40) * (0.40) = 0.16 or 16 percent. 3、If event A and event B cannot occur simultaneously, then events A and B are said to be: A) collectively exhaustive. B) statistically independent. C) mutually exclusive. D) conditionally dependent. The correct answer was C) If two events cannot occur together, the events are mutually exclusive. A good example is a coin flip: heads AND tails cannot occur on the same flip. 4、If two events are mutually exclusive, the probability that they both will occur at the same time is: A) 0.50. B) 1.00. C) 0.00. D) Cannot be determined from the information given. The correct answer was C) If two events are mutually exclusive, it is not possible to occur at the same time. Therefore, the P(A∩B) = 0. 5、Which of the following statements about probability is TRUE? A) A conditional probability is the probability that two or more events will happen concurrently. B) An outcome is the calculated probability of an event. C) Out of a sample of 100 widgets 10 were found to be defective, 20 were perfect, and 70 were OK. The probability of picking a perfect widget at random is 29 percent. D) An event is a set of one or more possible values of a random variable The correct answer was D) Conditional probability is the probability of one event happening given that another event has happened. An outcome is the numerical result associated with a random variable. The probability of picking a perfect widget is 20 / 100 = 0.20 or 20 percent. 6、There is a 40% chance that the economy will be good next year and a 60% chance that it will be bad. If the economy is good, there is a 50 percent chance of a bull market, a 30% chance of a normal market, and a 20% chance of a bear market. If the economy is bad, there is a 20% chance of a bull market, a 30% chance of a normal market, and a 50% chance of a bear market. What is the joint probability of a good economy and a bull market? A) 12%. B) 20%. C) 32%. D) 50%. The correct answer was B) Joint probability is the probability that both events, in this case the economy being good and the occurrence of a bull market, happen at the same time. Joint probability is computed by multiplying the individual event probabilities together: (0.40) * (0.50) = 0.20 or 20 percent. 7、What is the probability of a bull market next year? A) 32%. B) 50%. C) 12%. D) 20%. The correct answer was A) Because a good economy and a bad economy are mutually exclusive, the probability of a bull market is the sum of the joint probabilities of (good economy and bull market) and (bad economy and bull market): ((0.40)*(0.50)) + ((0.60)*(0.20)) = 0.32 or 32 percent. | 
发表于 2008-4-9 14:53
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