The actual ISO speed (formerly ASA) is a property of the film only. If you go to the actual ISO specification document, the number is based on exposure at the "toe" of a negative film, where it first starts to show density above the base level. The development details will affect strongly the slope of the curve above the toe, and therefore the exposure required to reach the standard 18% density grey level. Still the best description of how to rate film speed for specific applications can be found in Ansel Adams works on the Zone System.