How does sampling affect reliability?

Samples are collected from the population because sometimes populations are large enough to observe every value. Therefore, samples are collected from the population randomly and independently, so that, we can have a small picture of the population and we can make some inference about the entire population by analyzing those samples. If the samples are not collected randomly or from the appropriate population then there are fewer chances of obtaining a good picture of the population, thus, inappropriate inference and incorrect decision.

Here, the sample size is also very important because a large sample contains more values of the population, therefore, more chances that a large sample will represent the population in a better way in comparison to a small sample.