Frequency of exceedance any peak exceeding is . Its complement, is the probability of exceedance, the probability that has been exceeded at least once by time . This probability can be useful to estimate whether an extreme event will occur during a specified time period, such as the lifespan of a structure or the duration of an operation. If is small, for example for the frequency of a rare event occurring in a short time period, then Under this assumption, the frequency of exceedance is equal to the probability of exceedance per unit time, , and the probability of exceedance can be computed
Number density the objects in the volume "V" can be expressed as Similar expressions are valid for electric charge or any other extensive quantity associated with countable objects. For example, replacing "m" with "q" (total charge) and "m" with "q" (charge of each object) in the above equation will lead to a correct expression for charge. The number density of solute molecules in a solvent is sometimes called concentration, although usually concentration is expressed as a number of moles per unit volume (and thus called molar concentration). For any substance, the number density can be expressed in terms of its amount concentration