The publisher apologises for the error on page 6 of this article where an incorrect expression of covariances was given in the last line of Section 2 (the ninth line from the top of the page). The term should read:
![](https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary:20170131034759664-0026:S0373463316000631:S0373463316000631_eqnU1.gif?pub-status=live)
Published online by Cambridge University Press: 03 October 2016
The publisher apologises for the error on page 6 of this article where an incorrect expression of covariances was given in the last line of Section 2 (the ninth line from the top of the page). The term should read: