Cost-exchange ratio

In anti-ballistic missile defence the cost-exchange ratio is the ratio of the incremental cost to the aggressor of getting one addtional warhead through the defence screen, divided by the incremental cost to the defender of offsetting the additional missile. Generally, cost-exchange ratios favour the aggressor. The cost of running a few more missiles off the production line is relatively low compared to the costs incurred by the defence in neutralising them. The latter may involve expensive upgrades to radar and processing equipement as well as additional defensive missiles. Consideration of cost-exchange ratios was influential in persuading the United States and the Soviet Union to sign the ABM Treaty.