The previous example uses the Gaussian distribution for features that are essentially binary (UP = 1 and DOWN = 0) to represent the change in value. The mean value is computed as the ratio of the number of observations for which xi = UP over the total number of observations.
As stated in the first section, the Gaussian distribution is more appropriate for either continuous features or binary features for very large labeled datasets. The example is the perfect candidate for the Bernoulli model.