Quota sampling is random and it is normally used for interview surveys.

If you wish to offer statistical explanations, make statistical estimates or inferences about the target population from your sample or test a theory, you will choose probability sampling.

With all probability samples, it is not important that your sample size is large enough to provide you with the necessary confidence in your data.

The population is not the full set of cases or elements from which a sample is taken.

Statisticians have proved that the smaller the absolute size of a sample, the closer its distribution will be to the normal distribution and thus the more robust it will be.

For all research questions where it would be practicable for you to collect data from the entire population, you need to select a sample.

Without a sampling frame, you will have to consider using non- probability sampling.

A sample size of 30 or more will usually result in a sampling distribution for the mean that is very close to a normal distribution.  

For all research questions where it would be impracticable for you to collect data from the entire population, you need to select a sample.

Non-respondents are different from the rest of the target population because they are unable or unwilling to be involved in your research for whatever reason.