  In this study we critically examine some important papers on weak measurement
and weak values. We find some insufficiency and mistakes in these papers, and
we demonstrate that the real parts of weak values provide the back-action to
the post-selection, which is caused by weak measurement. Two examples, a
counterfactual statement of Hardy's paradox and experiments that determine the
average trajectory of photons passing through double slits, are investigated
from our view point.
