descriptive statistics：组织和总结信息，为自身（可以是population也可以是sample）审视和探索，
inferential statistics.从sample中推论population情况并评价推论可信度
在population中精挑细选出sample
Observational Studies:观察
and Designed Experiments：施加控制和影响，再观察
Observational studies can reveal only association（即事实一与事实二有关系）, whereas designed experiments can help establish causation（事实一导致了事实二）
