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Endgames Statistical Question

Snowball sampling

BMJ 2013; 347 doi: (Published 20 December 2013) Cite this as: BMJ 2013;347:f7511
  1. Philip Sedgwick, reader in medical statistics and medical education1
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, London, UK
  1. p.sedgwick{at}

Researchers examined the association between plasma HIV-1 RNA concentrations and the incidence of HIV in injecting drug users. A prospective cohort study design was used. Participants were injecting drug users, with or without HIV, followed up every six months between 1 May 1996 and 30 June 2007. Cohort members were recruited from an inner city community in Vancouver, Canada, using the method of snowball sampling. In total, 622 injecting drug users with HIV and 1429 injecting drug users without HIV were recruited.1

The researchers reported that in the community of injecting drug users, a longitudinal measure of plasma HIV-1 RNA concentration was positively correlated with the HIV incidence rate, independently of unsafe sexual behaviours and sharing used syringes. It was concluded the results could help inform HIV prevention and treatment interventions.

Which of the following statements, if any, are true? …

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