明天就考试，今晚我要等到ＮＮ给解答了这几个题．

xlyun

关于标准方差的，偶问了一次了没人给解答，偶最后问一次，明天就考试，不想留遗憾　　

1.     70, 75, 80, 85, 90, 105, 105, 130, 130, 130

The list shown consists of the times, in seconds, that it took each of 10 schoolchildren to run a distance of 400 meters.  If the standard deviation of the 10 running times is 22.4 seconds, rounded to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?

(A) One

(B) Two

(C) Three

(D) Four

(E) Five

 答案选B

2.      Resident in Town X hold a survey determine number of hours per week they spent in watching TV. The result of survey had a mean of 21 hours and standard deviation of 6 hours. The number of hour Pat spent in watching TV last week was between 1 and 2 standard deviation below the mean. Which of the following could be the number of hours Pat spent on TV last week?

A. 30     B. 20     C 18        D12

The answer is D

3.   A certain characteristic in a large population has a distribution that is symmetric about the mean m.  If 68 percent of the distribution lies within one standard deviation d of the mean, what percent of the distribution is less than m +
                d ?

 

A.      16%

B.      32%

C.      48%

D.     84%

E.      92%

The answer is d

neo1001

1.那十个数的平均数是100（我心算的，可能有些差异。。。），std 是22.4, 那么1 std below mean就是100-22.4=77.6

那么只有70和75这两个数是moren than 1 std below the mean

2.mean=21, std=6, 1 std below mean=21-6=15, 2 std below mean=21-6*2=9，所以Pat的时间在9和15之间

3. 画图，对称的分布，所以有50%大于m，50%小于m。另外，在[m-d, m+d]之间有68%，所以在[m, m+d]之间有68%/2=34％，所以，Prob([<=m+d])=Prob([<=m])+Prob([m, m+d])=50%+34%=84%

雨寒

明天我也考。。。

Good luck...

sundaysu

LZ,GOOD LUCK!

heslaw

good luck to all ^^

laipiyu

第一题还是不明白。how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?是什么意思？

1 standard deviation below the mean of the 10 running times是指比平均值低一个标准差的数值吗？

xlyun

谢谢大家!

偶今天下午考完了,V考的就是一拖shit.做第一篇RC时就感觉到进了低分区,哎没考出风格!

neo1001

以下是引用laipiyu在2008-4-10 23:39:00的发言：

第一题还是不明白。how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?是什么意思？

1 standard deviation below the mean of the 10 running times是指比平均值低一个标准差的数值吗？

就是这个意思

laipiyu

回答真是太快速了！！！！

不过还是有问题，不是问more than吗？75和70不是less than 77.6吗？是我对问题理解有误吗？

laipiyu

不好意思，刚刚读明白了。非常感谢！！！