说人们的题中通常是服从正态分布，标准差为1之内的有多少a(给了，忘了)
percent，标准差为2内的为95percent。问一个调查mean为18.6，则在6.8到12.6
之间为多少percent.
答(95-a)/2
From presented conditions, I guess a = 68.97%, Right? Anyone remembers -- please confirmed?
Because:
For Normal Distribution we talking about, it has certain statistical characteristics:
:: P(Probability) ={mean-stdev < X < mean+stdev} = 68.27%
:: P(Probability) ={mean-2stdev < X < mean+2stdev} = 95.45%
:: P(Probability) ={mean-3stdev < X < mean+3stdev} = 99.73%
This property indicates that for a specified Normal Distribution, the area from mean-3stdev to mean+3stdev covers 99.73% probability. Understand? (you must have learned from
lecture)
For this case, mean=18.6, stdev? Suppose the stdev=1, then, according to the conclusion from above, if a point has 99.73% probability falls in 15.6(18.6-3*1) to 21.6 (18.6+3*1); in other words, if a is within 10~12, we say "a" is not a member of this Normal Distribution, or, "a" is
just a small probability(<=0.7%) case. Right?
In general, if you wanna know the P(a
Normal Distribution Function Table.
::Conclusion
- This question may not be accurate, because for one Normal Distribution, it has only one stdev, while this question presented two? At the same time, 95% is so close to 95.45%, a ? Hence, I prefer to my guess.
- On the other hand, suppose the question is correct. The answer MUST be not correct. Anyway, (a-b)/2 cannot connect with 6.8~12.6?
- My comments are just from personal point of view. I have no right
to force you to select this explanation.