The approximation we just made is valid for as long as the uncertainty is smaller than the distance between the secondary marker and the EOF. In such case, the relative mobility uncertainty is directly proportional to the marker position uncertainty. When t A is too close to either t A → t EOF or t A → λt R the relative error induced in the computed mobility becomes very large. We can also compute the mobility uncertainty induced by that of t EOF , which we denote δt EOF . A similar computation yields