The sample variance s2 is easier to work with in the examples on pages 3 and 4 because it does not have square roots. The POPULATION VARIANCE σ2 is a PARAMETER of the population. The SAMPLE VARIANCE s2 is a STATISTIC of the sample. We use the sample statistic to estimate the population parameter The sample variance s2 is an estimate of the population variance σ2 Suppose we have a population with N individuals or items. Suppose that we want to take samples of size n individuals or items from that population IF we could list all possible samples of n items that could be selected from the population of N items, then we could find the sample variance for each possible sample. We would want the following to be true: We would want the average of the sample variances for all possible samples to equal the population variance. It seems like a logical property and a reasonable thing to happen. This is called “unbiased”
take:
- sample, population 一个样本一个整体
- 用所有样本平均，可以得到整体的值