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标题: 一道prep的DS [打印本页]

作者: youngerlee 时间: 2011-1-3 07:00 标题: 一道prep的DS

During an experiment, some water was removed from each of the 6 water tanks. If the standard deviation of the volumes of water in the tanks at the beginning of the experiment was 10 gallons, what was the standard deviation of the volumes of water in the tanks at the end of the experiment?

1) For each tank, 30% of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment.

2) The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.

作者: Puddings 时间: 2011-1-3 21:34

項數標準差,若同時增加或減少某數,例如加5,則標準差不變

項數標準差,若同時增加或減少某比例,例如5%,則標準差會等比增加或減少

作者: piggycandys 时间: 2011-1-4 06:52

答案是选一吧？A

作者: judyenglishS 时间: 2011-1-4 20:55

standard deviation

标准差是方差的开方，

方差s^2=[(x1-x)^2+(x2-x)^2+......(xn-x)^2]/n

如果每个值都下降30%，那么平均值同样下降30%，所以Xn-平均值就是不变的，（1）正确

从公式可知，如果只知道平均值不一样，而不知道每个值的变化，就不知道标准差是不是变化了。

这个题我也问过，公式是复杂了点，但其实不难

作者: youngerlee 时间: 2011-1-5 06:47

多谢！！

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