1. A method for developing an integrated circuit device in which mobility in a channel of a first transistor of the circuit will be modified by stress effects, the first transistor being disposed in a first diffusion region in a layout of the integrated circuit and there being a second diffusion region disposed outside and spaced from the first diffusion region in the integrated circuit layout, the method comprising: developing an approximation of stress-induced mobility variation in the channel of the first transistor, the approximation developed in dependence upon both a stress contribution arising due to a first materials interface at an edge of the first diffusion region, and a stress contribution arising due to a second materials interface at an edge of the second diffusion region; and providing the approximation for simulating the circuit.