  We show that measurements of the Neumann-to-Dirichlet map, roughly speaking,
on a certain part of the boundary of a smooth domain in dimension 3 or higher,
for inputs with support restricted to the other part, determine an electric
potential on that domain. Given a convexity condition on the domain, either the
set on which measurements are taken, or the set on which input functions are
supported, can be made to be arbitrarily small. The result is analogous to the
result by Kenig, Sj\"{o}strand, and Uhlmann for the Dirichlet-to-Neumann map.
The main new ingredient in the proof is a Carleman estimate for the
Schr\"{o}dinger operator with appropriate boundary conditions.
