# Probability Sampling

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A sample in which every possible [observation]] in the population has a known and finite ratio of probability of inclusion in the population relative to every other unit in the population. The most common cases of this are where:

- Each possible observation in the population has a known and non-zero probability of being included in the sample.
- Each possible observation has an unknown but equal probability of inclusion.

The most common variants of probability sampling used in survey research are: