Published in Int. J. Epidemiol. (2016) 45 (4): 1006-1010; doi:10.1093/ije/dyw103
This commentary was originally published with errors in an equation on p.1007 in section ‘Total and joint effects’.
The equation initially was shown as 
When it should have been 
This has now been corrected. The publishers apologise for the error.
