我刚刚开始学计量，对significance level 和confidence level 不太明白，自己想了很久，也去google搜索了wikipedia的解释，还是很迷惑！！ 明白的牛牛能不能给我解释解释啊？？
For example, a confidence interval can be used to describe how reliable survey results are. In a poll of election voting-intentions, the result might be that 40% of respondents intend to vote for a certain party. A 90% confidence interval for the proportion in the whole population having the same intention on the survey date might be 38% to 42%. From the same data one may calculate a 95% confidence interval, which might in this case be 36% to 44%. A major factor determining the length of a confidence interval is the size of the sample used in the estimation procedure, for example the number of people taking part in a survey.
1 这些数字是怎么得出来的？poll election voting intentions 的结果为什么是 normal distribution? 这又不是throw dice的均值问题，我觉得voting for election的问题就像扔硬币问题，非A即B啊？？不是吗？
2 即使是正态分布，根据z=(X-u)/std, 是怎样得出 38%-42%的? 在不知道sample size 的情况下。。
3 confidence level 的实际意义又是什么？和significance level的联系是什么？和为1？是就这样定义的？
我的逻辑有时候就是不给力啊。。。。。
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