参考文献：
http://www.statisticshowto.com/skewed-distribution/
left/negatively-skewed distributions ：
- boxplot：The left whisker will also be longer than the right whisker.
right/positively-skewed distributions
- the right whisker will be longer.
注意：median未必一定在mean和mode之间
对于too skewed的数据，如果需要进行参数检验（比如ANOVA），就需要用log变换，让它没那么skewed。
何时这么做？？
- Your data is highly skewed to the right (i.e. in the positive direction).
- The residual’s standard deviation is proportional to your fitted values
- The data’s relationship is close to exponential.
- You think the residuals reflect multiplicative errors that have accumulated during each step of the computation.