When some members of a population have a greater chance of being selected than others, the sampling is

When some members have a greater chance of selection than others, the sampling is non-probability sampling.

Selection probabilities are unknown or unequal, so representativeness cannot be assured.

Probability sampling instead gives every member a known, calculable chance of inclusion.

Simple random sampling grants strictly equal chances and is a probability method.

Systematic sampling picks every kth unit from a random start and is also a probability method.

Non-probability methods include convenience, quota, purposive and snowball sampling.