Types of Probability Samples

The advantage of probability sampling is that using statistics, one can estimate how closely the results of the survey will reflect the views of the whole population.

Random sampling avoids the pitfalls associated with convenience or purposeful sampling processes by limiting systematic bias. That does not mean that a random sample will necessarily accurately reflect the views of the population as a whole.

Any sample you select will differ from the population in some unknown ways. But, it will not be a systematic bias, such as would occur by only talking to freshman when studying the quality of general education on a college campus.