In some situations stratified random sampling may be more appropriate. You may have a population which can be divided up into a number of subsets (strata) such that the difference between units in different strata is much greater than the difference between units within each stratum. A probability sample may not have enough units from some of the smaller strata. A stratified random sample will ensure that each stratum is represented proportionally.
In other situations, cluster sampling may be more appropriate. Suppose you wish to visit a sample 1% of all schools in the country. If you were to choose the schools by probability sampling they would be all over the country and you would require a huge amount of time and money to visit them all. What you could do, instead, is to divide up the country into 1000 regions. Select 10 of these regions (1%) and then visit every school in the selected regions. Far less running around!
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the difference is just that non-probability sampling does not involve random selection, but probability sampling does.
Pros and Cons of a non-probability sampling
Non probability sampling and probability sampling are different because probability sampling uses random samples. Non probability sampling aren't random, but can still be representative of the population as a whole if done correctly.
an approach to sampling that has the characteristics of being randomly selected and the use of probability theory to evaluate sample results. Whereas non-statistical sampling is therefore any sampling approach that does not have both of the characteristicss of statistical sampling. I hope this will help....
The related web sites give a good idea of the types of non-random sampling. These include snowball, convenience, quota, self-selection, diversity, expert, and others. Non-randon sampling is usually done because it is less expensive, easier, and quicker than random sampling.