A statistic (or stat) in role-playing games is a piece of data that represents a particular aspect of a fictional character. That piece of data is usually a (unitless) integer or, in some cases, a set of dice. For some types of statistics, this value may be accompanied with a descriptive adjective, sometimes called a specialisation or aspect, that either describes how the character developed that particular score or an affinity for a particular use of that statistic (like Specialisations in Ars Magica or Attribute Aspects in Aria). Most games divide their statistics into several categories. The set of categories actually used in a game system, as well as the precise statistics within each category, vary greatly. The most often used types of statistic include: Attributes describe to what extent a character possesses natural, in-born characteristics common to all characters. Advantages and disadvantages are useful or problematic characteristics that are not common to all characters. Powers represent unique or special qualities of the character. In game terms, these often grant the character the potential to gain or develop certain advantages or to learn and use certain skills. Skills represent a character’s learned abilities in predefined areas. Traits are broad areas of expertise, similar to skills, but with a broader and usually more loosely defined scope, in areas freely chosen by the player. There is no standard nomenclature for statistics; for example, both GURPS and the Storytelling System refer to their statistics as “traits”, even though they are treated as attributes and skills. Many games make use of derived statistics whose values depend on other statistics, which are known as primary or basic statistics. Game-specific concepts such as experience levels, alignment, character class and race can also be considered statistics.