A sample proportion has a distribution

If a categorical data set is modelled as a random sample from a categorical population, the sample proportions must be treated as random quantities — they vary from sample to sample.

The population proportion in a category is called its probability, and is often denoted by π. The corresponding sample proportion is usually denoted by p.

  Sample Statistic Population Parameter
Mean µ
Standard deviation s σ
Proportion/probability p π

In practice, we only have a single sample and must use it to get information about the underlying population.