The Economist ranked 32 countries according to nuclear security, measured by their ability to safely contain nuclear material which could potentially be used in the creation of nuclear weapons.  The 32 countries included are those which possess over one kilogram of the type of nuclear material used to construct weapons.  The nuclear security index was developed by the Economist Intelligence Unit and the Nuclear Threat Initiative.  According the the Nuclear Threat Initiative, measures of nuclear security include "indicators in physical protection, control and accounting, personnel and security infrastructure, security during transport and response capabilities."  Countries with nuclear weapons tend to rank lower on nuclear security due to the fact that they are in possession of larger quantities of nuclear materials.  Britain received a higher score on the nuclear security index than any other country known to possess nuclear weapons.