  An overall reduction factor (ORF) is introduced for studying the quenching of
single particle strengths through nucleon transfer reactions. The ORF includes
contributions of all the probed bound states of the residual nucleus in a
transfer reaction and permits a proper comparison with results of inclusive
knockout reactions. A systematic analysis is made with 103 sets of angular
distribution data of $(p,d)$ reactions on 21 even-even targets with atomic mass
numbers from 8 to 56 using the consistent three-body model reaction methodology
proposed in [J. Lee, J.A. Tostevin, B.A. Brown, et al., Phys. Rev. C 73, 044608
(2006)]. The extracted ORFs are found to be nearly independent on the nuclear
isospin asymmetry, which is different from the systematics of inclusive
knockout reactions but is consistent with the recent measurement of $(d,t)$,
$(d,3He)$, $(p,2p)$, and $(p,pn)$ reactions on nitrogen and oxygen isotopes and
\textit{ab initio} calculations.
