  In systems belonging to the universality class of the random field Ising
model, the standard hyperscaling relation between critical exponents does not
hold, but is replaced by a modified hyperscaling relation. As a result,
standard formulations of finite size scaling near critical points break down.
In this work, the consequences of modified hyperscaling are analyzed in detail.
The most striking outcome is that the free energy cost \Delta F of interface
formation at the critical point is no longer a universal constant, but instead
increases as a power law with system size, \Delta F proportional to $L^\theta$,
with $\theta$ the violation of hyperscaling critical exponent, and L the linear
extension of the system. This modified behavior facilitates a number of new
numerical approaches that can be used to locate critical points in random field
systems from finite size simulation data. We test and confirm the new
approaches on two random field systems in three dimensions, namely the random
field Ising model, and the demixing transition in the Widom-Rowlinson fluid
with quenched obstacles.
