  In the analysis of elastic-scattering experimental data, optical-model
parameters (usually, depths of real and imaginary potentials) are fitted and
conclusions are drawn analyzing their variations at bombardment energies close
to the Coulomb barrier (threshold anomaly). The judgement about the shape of
this variation (related to the physical processes producing this anomaly)
depends on these fitted values but the robustness of the conclusions strongly
depends on the uncertainties with which these parameters are derived. We will
show that previous published studies have not used a common criterium for the
evaluation of the parameter uncertainties. In this work, a study of these
uncertainties is presented, using conventional statistic tools as well as
bootstrapping techniques. As case studies, these procedures are applied to
re-analyze detailed elastic-scattering data for the $^{12}$C + $^{208}$Pb and
the $^6$Li + $^{80}$Se systems.
