下礼拜要考P...超紧张... 这边有个SAMPLE中关於negative binominal的问题: 希望有大大帮忙解惑 A company takes out an insurance policy to cover accidents that occur at its manufacturing plant. The probability that one or more accidents will occur during any given month is 3/5. The number of accidents that occur in any given month is independent of the number of accidents that occur in all other months. Calculate the probability that there will be at least four months in which no accidents occur before the fourth month in which at least one accident occurs. 解答提供其中一个做法: 4! 5! 6! (2/5)^4 + -------*[(2/5)^4]*(3/5)+-------*[(2/5)^4]*(3/5)^2+-------*[(2/5)^4]*(3/5)^3=0.2898 3!*1! 3!*2! 3!*3! which can be derived directly or by regarding the problem as a negative binomial distribution with i) success taken as a month with no accidents ii) k = the number of failures before the fourth success, and iii) calculating Pr[k>=3] 我不是很懂为什麽要让2/5每一项都要到四次方 想很久还是想不懂 麻烦大家了 感激不尽... --

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