  In this work we revisit the important and controversial concept of quantum
weak values, aiming to provide a simplified understanding to its associated
physics and the origin of anomaly. Taking the Stern-Gerlach setup as a working
system, we base our analysis on an exact treatment in terms of quantum Bayesian
approach. We also make particular connection with a very recent work, where the
anomaly of the weak values was claimed from the pure statistics in association
with "disturbance" and "post-selection", rather than the unique quantum nature.
Our analysis resolves the related controversies through a clear and
quantitative way.
