Question

Why should you generally expect some error when estimating a parameter? (for example, a population? mean) by a statistic? (for example, a sample? mean)? what is this kind of error? called?

Answer 1

This is called as sampling error or sampling variability. Sampling error is the variability that happens because the value of a sample statistic varies from sample to sample. Sampling error is acquired when a population is estimated from a subset, or sample.