. 2011 Apr 2;33(4):503–510. doi: 10.1093/pubmed/fdr029
© The Author 2011, Published by Oxford University Press on behalf of Faculty of Public Health. All rights reserved
This article is made available via the PMC Open Access Subset for unrestricted re-use and analyses in any form or by any means with acknowledgement of the original source. These permissions are granted for the duration of the COVID-19 pandemic or until permissions are revoked in writing. Upon expiration of these permissions, PMC is granted a perpetual license to make this article available via PMC and Europe PMC, consistent with existing copyright protections.