A probability is a numerical description of how likely an event is to occur. The probability of an event is a number between 0 and 1, where, roughly speaking, 0 indicates impossibility of the event and 1 indicates certainty (at least when the number of possible events is finite, as is the case in most role-playing game contexts). The higher the probability of an event, the more likely it is that the event will occur.
As an example, consider the rolling of a d10 (a ten-sided dice). Since we assume that all random generators (such as dice) in RPGs are fair, the ten possible outcomes (the integers from "1" to "10") are equally probable; the probability of "1" equals the probability of "2" equals the probability of "3" and so on; and since no other outcomes are possible, the probability of any specific result is 1/10 (which could also be written as 0.1 or 10%). Probabilities can be numerically described by the number of "desired" outcomes divided by the total number of all outcomes.
A related concept is the probability distribution of an experiment (such as rolling a die), which is a mathematical function that gives the probabilities of occurrence of different possible outcomes for an experiment. It is a mathematical description of a random phenomenon in terms of its sample space and the probabilities of events (subsets of the sample space).
A single die roll results in what is called a flat distribution. That is, each result has an equal result of occurring. However, increasing the number of dice will make the probability distribution closer to a normal distribution (bell curve). For 2d6, for example, only one result out of thirty-six possibilities is a 2, but there are six different ways of adding up to 7. That means a roll of 2d6 (summed) is six times more likely to produce a 7 than a 2. 7 is the mode, or most common result. Increasing numbers of dice have a stronger central tendency and less very high or very low numbers. Larger dice have more very high or very low numbers.
External links[]
- Probability at Wikipedia, the free encyclopedia.
- AnyDice, an online web app for calculating probability distributions when rolling different dice or dice pools.