我們常利用抽樣的方法取得少數與母體有關係的資料, 再利用這些樣本來做後續的統計分析, 所以我們關心的不只是樣本本身是否具有代表性, 更想由其中的特性來得知母體的資訊, 在這之下, 樣本的分配情況就值得加以研究.

- parameter
- A numerical of a population is called a
**parameter**. - statistic
- A sample-based quantity is called a
**statistic**.

A**statistic**is a numerical valued function of the sample observations.

For example, the sample mean is a statistic because its numerical value can be computed once the sample data, consisting of the values of*X*_{1},...*X*_{n}, are available. Likewise, the sample median and the sample standard deviation are also sample-based quantities so each is a statistic.

A sample-based quantity (statistic) must serve as our source of information about the value of a parameter. Three points are crucial:

- 1.
- Because a sample is only a part of the population, the numerical value of a statistic cannot be expected to give us the exact value of the parameter.
- 2.
- The observed value of a statistic depends on the particular sample that happens to be selected.
- 3.
- There will be some variability in the values of a statistic over different occasions of sampling.