# 同学们，距离AP考试还剩9周了，刺不刺激！

2018年5月7日AP考试只剩

9周啦！

standard error of the mean

&

error bar

Where SEx = standard error, s=standard deviation, n= the sample size

The standard error tells us how certain we can be that our mean value is the true mean for the population that we have sampled. We can be 95% certain that—if we took a second sample from the same population—the mean for that second sample would lie within 2×our value of SE from the mean for our first sample.

If we draw an error that extends two standard errors above the mean and two standard errors below it, then we can be 95% certain that the true value of the mean lies within this range.

We can use these error bars to help us to decide whether or not there is significant difference between these three populations. If the error bars overlap, then the difference between the two groups is definitely not significant. If the error bars do not overlap, we still cannot be sure that the difference is significant—but at least we know it is possible that it is.

https://pan.baidu.com/s/1r2UNS8sljrRjYt8uj6Ep8Q

(chi-squared test)

Chi-squared test allows us to compare our observed results with the expected results, and decide whether or not there is a significant difference between them.