This page describes the calculation for hit chances with falloff.
Many of the EW modules have a falloff range.
Any module is 100% effective from 0 to optimal. At optimal+falloff the effectiveness drops to 50% and at optimal + 2*falloff it is about 2%. This means that at optimal+falloff there is a 50% chance that the module will fail and not do anything. To determine the chance of success in falloff range, we use the following formula:
C = 0.5^((R-O)/F)^2)
This gives us an S curve:
Where R is the range from you to the target, O the optimal range of the module, F the falloff range of the module and C the chance of success. As a result, you will get a multiplier. This is a chance multiplier. In case of jammers, just multiply the calculated chance by it. In case of other modules, this will simply show you the chances of the module succeeding at a given range.
Guns use a similar calculation for hit chances, however they are also affected by signal radius and tracking speed. CCP's own tracking guide is probably the best place to look for more information on this: Tracking Guide