title: Using expert opinion to quantify unmeasured confounding bias parameters.
abstract: To develop and apply a method to quantify bias parameters in the case example of the association between alcohol use and HIV-serodiscordant condomless anal sex with potential confounding by sensation seeking among men who have sex with men (MSM), using expert opinion as an external data source.
  
  
  
  Through an online survey, we sought the input of 41 epidemiologist and behavioural scientists to quantify six parameters in the population of MSM: the proportion of high sensation seeking among heavy-drinking MSM, the proportion of sensation seeking among low-level drinking MSM, and the risk ratio (RR) of the association between sensation seeking and condomless anal sex, for HIV-positive and HIV-negative MSM.
  
  
  
  Eleven experts responded. For HIV-positive heavy drinkers, the proportion of high sensation seeking was 53.6% (beta distribution [α=5.50, β=4.78]), and 41.1% (beta distribution [α=3.10, β=4.46]) in HIV-negative heavy drinkers. In HIV-positive low-level alcohol drinkers, high sensation seeking was 26.9% (beta distribution [α=1.81, β=4.92]), similar to high sensation seeking among HIV-negative low-level alcohol drinkers (25.3%) (beta distribution [α=2.00, β=5.89]). The lnRR for the association between sensation seeking and condomless anal sex was ln(2.4) (normal distribution [μ=0.889, σ=0.438]) in HIV-positive and ln(1.5) (normal distribution [μ=0.625, σ=0.391]) in HIV-negative MSM.
  
  
  
  Expert opinion can be a simple and efficient method for deriving bias parameters to quantify and adjust for hypothesized confounding. In this test case, expert opinion confirmed sensation seeking as a confounder for the effect of alcohol on condomless anal sex and provided the parameters necessary for probabilistic bias analysis.
authors.affiliation: Global Health Sciences, University of California, San Francisco, CA, USA.|Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, PO Box: 14155-6446, Tehran, Iran.
authors.forename: Soodabeh
authors.initials: S
authors.lastname: Navadeh
authors.affiliation: Global Health Sciences, University of California, San Francisco, CA, USA.|Department of Epidemiology and Biostatistics, University of California, San Francisco, CA, USA.
authors.forename: Ali
authors.initials: A
authors.lastname: Mirzazadeh
authors.affiliation: Department of Epidemiology and Biostatistics, University of California, San Francisco, CA, USA.|San Francisco Department of Public Health, San Francisco, CA, USA.
authors.forename: Willi
authors.initials: W
authors.lastname: McFarland
authors.affiliation: Department of Medicine, Center for AIDS Prevention Studies, University of California, San Francisco, CA, USA.
authors.forename: Sarah
authors.initials: S
authors.lastname: Woolf-King
authors.affiliation: Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, PO Box: 14155-6446, Tehran, Iran. mansournia_ma@yahoo.com.
authors.forename: Mohammad Ali
authors.initials: MA
authors.lastname: Mansournia
mesh_terms: D000428:Alcohol Drinking / Q000453:epidemiology / Q000523:psychology*; D015982:Bias; D002170:Canada / Q000453:epidemiology; D017280:Condoms / Q000706:statistics & numerical data; D005240:Feasibility Studies; D018023:HIV Seronegativity; D006679:HIV Seropositivity / Q000453:epidemiology / Q000523:psychology*; D018451:Homosexuality, Male / Q000523:psychology* / Q000706:statistics & numerical data; D006801:Humans; D008297:Male; D012309:Risk-Taking; D012677:Sensation; D012725:Sexual Behavior / Q000523:psychology; D011795:Surveys and Questionnaires; D047568:Unsafe Sex / Q000523:psychology*