Abstract
We study the multifractal analysis (MFA) of electronic wavefunctions at the metal to insulator transition in the 3D Anderson model for very large system sizes up to 240^3. The singularity spectrum f(alpha) is numerically obtained using the typical and ensemble averages of the scaling law for the generalized inverse participation ratio, employing box-size and system-size scaling. The validity of the symmetry law in the multifractal spectrum is carefully analysed at the MIT. System-size scaling with ensemble average appears as the most adequate method to carry out the numerical MFA.