Follow height rules when installing a distribution box. Wall-mounted boxes should be 4. In probability theory and statistics, a normal distribution or Gaussian distribution is a type of continuous probability distribution for a real-valued random variable. This guide covers its definition, probability rules. Normal Distribution is the most common or normal form of distribution of Random Variables, hence the name "normal distribution. As there are 30 units (from zero to 30), then the height is 1/30.