Probability sampling provides a scientific technique of drawing samples from the population according to some laws of chance, in which each unit in the universe has some definite pre-assigned probability of being selected in the sample.
Different types of sampling are in which:
(i) Each sample unit has an equal chance of being selected.
(ii) Sampling units have varying probability of being selected.
(iii) Probability of selection of a unit is proportional to the sample size.