The Publisher regrets the inclusion of an error introduced during the production process of the above article.

An equation was originally published with erroneous ampersands. The correct version of the equation is below, and the original article has been updated to reflect this.

(1) \begin{align}&\hat \tau \equiv \left\{ {\left( {{{\sum\limits_{i = 1}^N {{Y_{i2}}} {D_i}{T_i}} \over {\sum\limits_{i = 1}^N {{D_i}} {T_i}}} - {{\sum\limits_{i = 1}^N {{Y_{i1}}} {D_i}{T_i}} \over {\sum\limits_{i = 1}^N {{D_i}} {T_i}}}} \right)} \right. - \left. {\left( {{{\sum\limits_{i = 1}^N {{Y_{i2}}} (1 - {D_i}){T_i}} \over {\sum\limits_{i = 1}^N {(1 - {D_i})} {T_i}}} - {{\sum\limits_{i = 1}^N {{Y_{i1}}} (1 - {D_i}){T_i}} \over {\sum\limits_{i = 1}^N {(1 - {D_i})} {T_i}}}} \right)} \right\}\\ &{ - \left\{ {\left( {{{\sum\limits_{i = 1}^N {Y_{i2}}{D_i}(1 - {T_i})} \over {\sum\limits_{i = 1}^N {D_i}(1 - {T_i})}} - {{\sum\limits_{i = 1}^N {Y_{i1}}{D_i}(1 - {T_i})} \over {\sum\limits_{i = 1}^N {D_i}(1 - {T_i})}}} \right)} \right. - \left. {\left( {{{\sum\limits_{i = 1}^N {Y_{i2}}(1 - {D_i})(1 - {T_i})} \over {\sum\limits_{i = 1}^N (1 - {D_i})(1 - {T_i})}} - {{\sum\limits_{i = 1}^N {Y_{i1}}(1 - {D_i})(1 - {T_i})} \over {\sum\limits_{i = 1}^N (1 - {D_i})(1 - {T_i})}}} \right)} \right\}}\end{align}