  We present a method for assessing the sensitivity of the true causal effect
to unmeasured confounding. The method requires the analyst to specify two
intuitive parameters. Otherwise, the method is assumption-free. The method
returns an interval that contains the true causal effect. Moreover, the bounds
of the interval are sharp, i.e. attainable. We show experimentally that our
bounds can be sharper than those obtained by the method of Ding and VanderWeele
(2016). Finally, we extend our method to bound the natural direct and indirect
effects when there are measured mediators and unmeasured exposure-outcome
confounding.
