  It is extremely important to devise a reliable method to extract
spectroscopic factors from transfer cross sections. We analyse the standard
DWBA procedure and combine it with the asymptotic normalisation coefficient,
extracted from an independent data set. We find that the single particle
parameters used in the past generate inconsistent asymptotic normalization
coefficients. In order to obtain a consistent spectroscopic factor,
non-standard parameters for the single particle overlap functions can be used
but, as a consequence, often reduced spectroscopic strengths emerge. Different
choices of optical potentials and higher order effects in the reaction model
are also studied. Our test cases consist of: $^{14}$C(d,p)$^{15}$C(g.s.) at
$E_d^{lab}=14$ MeV, $^{16}$O(d,p)$^{17}$O(g.s.) at $E_d^{lab}=15$ MeV and
$^{40}$Ca(d,p)$^{41}$Ca(g.s.) at $E_d^{lab}=11$ MeV. We underline the
importance of performing experiments specifically designed to extract ANCs for
these systems.
