The probability of an attack is based on the total nuclear stockpiles and the following table:

Total Nuclear Weapons: | 0 | 1-5 | 6-11 | 12-18 | 19-27 | >28 |

Attack Probability | 0 | 1/32 | 1/16 | 1/8 | 1/4 | 1/2 |

The simulation starts at a 1 in 4 chance, which is based on the “pessimistic” probability estimate of nuclear expert Richard Garwin referred to in the reading. [He estimated a 50% chance in 10 years, corresponding to a 24% chance in 4 years]. The attack probability can be reduced to 1 in 8 before the first possibility of an attack if the global stockpile decreases by 2 in Round 1.

Whether an attack occurs is determined by the flip of a coin. If the odds are 1 in 4, flip the coin twice; if it comes up heads both times an attack has occurred (for 1 in 8, 3 heads in a row signifies an attack, 1 in 16, 4 heads in a row, etc.). Any result beside all heads means that no attack has occurred this round. If all heads come up, you must then determine which country has suffered an attack. This is done by rolling a die and looking at the Geopolitical Power Rankings.

Countries with a lower GPR, being perceived as more powerful, will have a higher probability of suffering the terrorist attack. If the die rolls 1, 2, or 3, it hits the country with the lowest GPR; if the die rolls 4 or 5, it hits the country with the second-lowest GPR; and if the die rolls 6, it hits the country with the third-lowest GPR. Any ties can be broken by rolling the die again, e.g. if 6 is rolled, meaning the third-lowest nation is hit, and two countries are tied for this ranking, then assign 1, 2, or 3 to one of the countries and 4, 5, or 6 to the other and roll again.

Instructors should be aware that the determination of an attack does not have to be random. As keepers of the coin and die they can orchestrate or omit an attack at their discretion.