The term probability samplingis used when the selection of the sample is purely based on chance. The human mind has no control on the selection or non- selection of the units for the sample. Every unit of the population has known nonzero probability of being selected for the sample. The probability of selection may b equal or unequal but it should be non-zero and should be known. The probability samplingis also called the random sampling (not simple random sampling). Some examples of random sampling are:
In non-probability sampling,the sample is not based on chance. It is rather determined by some person. We cannot assign to an element of population the probability of its being selected in the sample. Somebody may use his personal judgment in the selection of the sample. In this case the sampling is called judgment sampling.A drawback in non-probability sampling is that such a sample cannot be used to determine the error. Any statistical method cannot be used to draw inference from this sample. But it should be remembered that judgment sampling becomes essential in some situations. Suppose we have to take a small sample from a big heap of coal. We cannot make a list of all the pieces of coal. The upper part of the heap will have perhaps big pieces of coal. We have to use our judgment in selecting a sample to have an idea about the quality of coal. The non- probability sampling is also called non-random sampling.