  The probability density function (PDF) for critical wavefunction amplitudes
is studied in the three-dimensional Anderson model. We present a formal
expression between the PDF and the multifractal spectrum f(alpha) in which the
role of finite-size corrections is properly analyzed. We show the non-gaussian
nature and the existence of a symmetry relation in the PDF. From the PDF, we
extract information about f(alpha) at criticality such as the presence of
negative fractal dimensions and we comment on the possible existence of
termination points. A PDF-based multifractal analysis is hence shown to be a
valid alternative to the standard approach based on the scaling of general
inverse participation ratios.
