Question

Why is varability in sampling (the fact that samples from the same population taken using the same method will often be so different) so significant?

Answer 1

When samples are taken from the same population using the same method, each unit of a population has a pre-assigned positive probability of getting selected in the sample. This probability remains the same as the method is the same. So, different units will be included in the different samples from the same population since a sample consists of only apart of the population. Thus there is at least one different population unit in each sample which results in the variability in sampling. If the sample size is small, then the samples cover a small part of the population & thus have greater variability. As the sample size increases the samples covers more part of the population & hence have smaller variability.