Given a set of random variables *X _{1} ... X_{n}*, an

The * joint probability distribution* is a table that assigns
a probability to each atomic event. Any question of conditional
probability can be answered from the joint.[Example from
Russell & Norvig.]

Toothache | ¬ Toothache | |

Cavity | 0.04 | 0.06 |

¬ Cavity | 0.01 | 0.89 |

** Problems:**

- The size of the table is combinatoric: the product of the
number of possibilities for each random variable.
- The time to answer a question from the table will also
be combinatoric.
- Lack of evidence: we may not have statistics for some table entries, even though those entries are not impossible.