The first displayed (unnumbered) equation on page 11 in Abbring and van den Berg (2004) gives an expression for Pr(Tm > t, Tp > tp|x) that is incorrect. Under the assumption (implicit in the paper) that the distribution of Tp|x is not defective, (Formula presented.) If tp ≥ t, this does not depend on the treatment effect Δ(t|tp, x) and gives the expression on page 11. If tp < t, however, the expression on page 11 is incorrect. This is an isolated and inconsequential error in the sense that the equation is not referred to elsewhere in the paper and does not affect the text.We are grateful to Sumedha Gupta for drawing our attention to this error (first online draft of this erratum dated 2011).