All sampling units in the study population have
a known, non-zero probability of being selected
in the sample, usually through a randomized process.
This allows for inferences to be made about the
population based on the sample.
The sampling units are selected through a non-randomized
process, therefore the probability of selecting
any sampling unit is not known. Non-probability
sampling may be biased. Results may not be generalizable
to population of interest.