But note that nothing in Eq. 5.3 forces z to be a legal probability, that is, to lie between 0 and 1. In fact, since weights are real-valued, the output might even be negative; z ranges from −∞ to ∞. To create a probability, we'll pass z through the sigmoid function, σ (z). The sigmoid sigmoid function (named because it looks like an s) is also called the logistic function, and gives logistic regression its name. The sigmoid has the following equation,