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Sampling methods are used to draw samples from a population, ensuring that they are drawn without bias and will accurately represent the population. 

The random sampling method is a commonly used method of sample collection. Here, each member of the population has an equal chance of being selected.

Consider a container with twenty plastic balls, five balls each of four different colors—red, black, blue, and green. 

If a ball is picked up randomly from the box, there is a 5 out of 20, or 1 in 4, chance that the ball is red, black, blue, or green. So, each color has an equal chance of being selected. 

The probability sampling method is similar to random sampling but with one significant difference. Each member of the population has a known, but not necessarily the same, chance of being selected.

Suppose the same box is filled with different numbers of colored balls. Each colored ball has a fixed but not necessarily an equal chance of being selected.