5.1Probability Basics
uncertainty is inherent in inferential statistics，因为总是需要样本估计总体，The science of uncertainty is called probability theory.学习概率分布帮助我们通过样本分析总体的相关数据（eg，mean。。。）
Basically, by an experiment, we mean an action whose outcome cannot be predicted with certainty
By an event, we mean some specifified result that may or may not occur when an experiment is performed
More generally, the frequentist interpretation of probability construes the probability of an event to be the proportion of times it occurs in a large number of repetitions of the experiment.(即用概率代表一个事件在大量重复实验中（即sample中有大量重复test）发生的比例）
两组不同的扔硬币实验，每组样本数是100，每一次计算样本中正面朝上的相关频率，即第一次100%（1/1），第二次50%（1/2），第三次33%（1/3）等
Although the frequentist interpretation is helpful for understanding the meaning of probability, it cannot be used as a defifinition of probability. One common way to defifine probabilities is to specify a probability model—a mathematical description of the experiment based on certain primary aspects and assumptions.
因此，比如假设扔硬币是二项分布
The equal-likelihood model：假设每个结果（不同）发生的概率相同。
The equal-likelihood model discussed earlier in this section is an example of a probability model. Its primary aspect and assumption are that all possible outcomes are equally likely to occur. We discuss other probability models later in this and subsequent chapters
