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13.2 Argument order and the vertical bar

Each probability function has a specific outcome value and a number of parameters. Following conditional probability notation, probability density and mass functions use a vertical bar to separate the outcome from the parameters of the distribution. For example, normal_lpdf(y | mu, sigma) returns the value of mathematical formula \(\log \text{Normal}(y \, | \, \mu, \sigma)\). Cumulative distribution functions separate the outcome from the parameters in the same way (e.g., normal_lcdf(y_low | mu, sigma)