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Wand H, Guy R, Donovan B et al. Population attributable risk for chlamydia infection in a cohort of young international travellers (backpackers) and residents in Australia. BMJ Open 2011;1:e000004.

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10. Wand H, Ramjee G. Combined impact of sexual risk behaviors for HIV seroconversion among women in Durban, South Africa: implications for prevention policy and planning. AIDS Behav 2011;15:479–86.

References in the manuscript should be re-numbered accordingly.

‘Introduction’ section, page 2, paragraph 1, lines 8-12 should have read: “In this paper, we use the population attributable risk (PAR), which takes account of both the odds ratio of specific risk factors and their prevalence in the population, to provide a quantitative assessment of the potential impact of reducing a risk factor on disease incidence at a population level.10

The ‘Statistical analysis’ section, ‘Population attributable risk’ sub-section, page 2, paragraph 2, lines 2–5 should have read: “The PAR is formulated as a function of odds ratio (OR) (s) and the prevalence (p) (s) of the risk factor(s). When there is only one risk factor at two levels (1 versus 0).10

The ‘Statistical analysis’ section, ‘Population attributable risk’ sub-section, page 2, paragraph 3, lines 1-3 should have read: “Where OR is the odds ratios, p is the prevalence of the risk factor in the population and s indexes the two strata determined by the value of the risk factor.10

The ‘Statistical analysis’ section, ‘Population attributable risk’ sub-section, page 2, paragraph 4, lines 1-6 should have read: “where OR s and p s, s=1,…,S, are the odds ratios and the prevalences in the target population for the sth combination of the risk factors. Full PAR can be estimated by using Equation and interpreted as the percent reduction expected in the number of HIV seroconversion if all the known risk factors were eliminated from the target population. In a multifactorial disease setting, at least some key risk factors such as age and sex are not modifiable. This limits the practical utility of the full PAR which is based on modification of all variables of interests. In an evaluation of a preventive intervention in a multifactorial disease setting, the interest is in the percent of cases associated with the exposures to be modified, when other risk factors, particularly non- modifiable, exist but do not change as a result of the intervention. Therefore we derived and used partial PAR which kept unmodifiable variable(s) unchanged10”.

The ‘Statistical analysis’ section, ‘Population attributable risk’ sub-section, page 2, paragraph 5, should have read: “where t denotes a stratum of unique combinations of levels of all background risk factors which are not modifiable and/or not under study, t=1,…,T and OR2t is the odds ratio in combination t relative to the lowest risk level, where OR 2,1=1. As previously, t indicates a risk factor defined by each of the unique combinations of the levels of the modifiable risk factors, that is, those risk factors to which the PAR applies, s=1,…,S, and OR1 is the relative risk corresponding to combinations relative to the lowest risk combination, OR110

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