The four possible outcomes that could occur if you flipped a
coin twice are listed in Table 1. Note that
the four outcomes are equally likely: each has probability

Outcome | First Flip | Second Flip |
---|---|---|

1 | Heads | Heads |

2 | Heads | Tails |

3 | Tails | Heads |

4 | Tails | Tails |

The four possible outcomes can be classifid in
terms of the number of heads that come up. The number could be
two (Outcome 1), one (Outcomes 2 and 3) or 0 (Outcome 4). The
probabilities of these possibilities are shown in Table 2 and in Figure 1. Since two of the
outcomes represent the case in which just one head appears in
the two tosses, the probability of this event is equal to

Number of Heads | Probability |
---|---|

0 | 1/4 |

1 | 1/2 |

2 | 1/4 |

Figure 1 is a discrete probability distribution: It shows the probability for each of the values on the X-axis. Defining a head as a "success," Figure 1 shows the probability of 0, 1, and 2 successes for two trials (flips) for an event that has a probability of 0.5 of being a success on each trial. This makes Figure 1 an example of a binomial distribution.