The Global Terrorism Index ranks 163 countries based on four indicators weighted over five years. A country’s annual Global Terrorism Index score is based on a unique scoring system to account for the relative impact of incidents in the year.
The four factors counted in each country’s yearly score are:
(1) total number of terrorist incidents in a given year
(2) total number of fatalities caused by terrorists in a given year
(3) total number of injuries caused by terrorists in a
given year
(4) total number of hostages caused by terrorists in a given year

Each of the factors is weighted between zero and three, and a five year weighted average is applied in a bid to reflect the latent psychological effect of terrorist acts over time.