An *unconditional, *or* marginal, probability* is one where the events (possible outcomes) are independent of each other. When you create a *joint probability table*, the unconditional probability of an event appears as a row total or a column total.

For example, say that you create a joint probability table representing the distribution of students in a business school; you classify them according to major and whether they're working on a bachelor's degree or a master's degree.

Degree Type |
Majoring in Finance |
Majoring in Accounting |
Majoring in Marketing |
Total |
---|---|---|---|---|

Bachelor's degree |
0.26 | 0.36 | 0.18 | 0.80 |

Master's degree |
0.09 | 0.07 | 0.04 | 0.20 |

Total |
0.35 |
0.43 |
0.22 |
1.00 |

Based on the table, you define the following events:

*B*= pursuing a bachelor's degree*M*= pursuing a master's degree*F*= majoring in finance*A*= majoring in accounting*T*= majoring in marketing

You can then find the unconditional probabilities of the following events directly from the table:

*P*(*B*) = the probability of pursuing a bachelor's degree*P*(*M*) = the probability of pursuing a master's degree*P*(*F*) = the probability of majoring in finance*P*(*A*) = the probability of majoring in accounting*P*(*T*) = the probability of majoring in marketing

Say you want to find the probability that a randomly chosen business student is pursuing a bachelor's degree. In other words, you want to calculate *P*(*B*).

Referring to the table, you look at the first row (which refers to students pursuing their bachelor's degrees). The row total is 0.80. This is the probability that a randomly chosen student is pursuing a bachelor's degree.

Suppose you want to know the probability that a randomly chosen student is majoring in finance. In other words, you want to calculate *P*(*F*).

Referring to the table, you look at the first column (which refers to students majoring in finance). The column total is 0.35. This is the probability that a randomly chosen student is majoring in finance.

You can find the remaining unconditional probabilities in the same way. These are:

*P*(*M*) = 0.20

*P*(*A*) = 0.43

*P*(*T*) = 0.22