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Table 1 Compositional bias of the follow-up sample based on the behavioural risk factors associated with HIV in the baseline study

From: HIV incidence from the first population-based cohort study in India

   Proportion having this risk factor   Relative impact on HIV estimationb  
Variables Baseline HIV prevalence Among those who did not participate in follow-up Among those who participated in follow-up Relative impact of risk factor on HIV per unit populationa Among those who did not participate in follow-up Among those who participated in follow-up Ratio of participated to not participatedc
Urban men (Number not participated, number participated)        
Not circumcised (831, 2227) 3.95 0.806 0.725 1.00 3.184 2.864  
More than one lifetime woman sex partners or ever visited sex worker (1127, 1815) 4.23 0.433 0.440 0.76 1.392 1.413  
More than two lifetime women sex partners or ever visited sex worker (1131, 1817) 4.49 0.333 0.341 0.58 0.868 0.887  
Have sex after consuming alcohol (1128, 1816) 5.41 0.234 0.230 0.58 0.734 0.721  
Had tattooing (1145, 1841) 7.45 0.076 0.088 0.27 0.153 0.177  
Used any recreational drugs (1131, 1798) 14.29 0.016 0.010 0.13 0.030 0.019  
Men who had sex with men (830, 2226) 11.25 0.020 0.023 0.13 0.029 0.034  
Had blood transfusion (1145, 1841) 8.08 0.029 0.029 0.10 0.023 0.023  
Overall urban men 2.45     6.413 6.138 0.957
Rural men (Number not participated, number participated)        
More than two lifetime women sex partners or ever visited sex worker (830, 2226) 2.83 0.380 0.412 0.44 0.473 0.514  
More than one lifetime woman sex partner or ever visited sex worker (830, 2226) 2.28 0.525 0.574 0.40 0.479 0.523  
Have sex after consuming alcohol (830; 2,225) 3.20 0.246 0.235 0.28 0.220 0.211  
Not circumcised (1131, 1816) 1.97 0.915 0.909 0.13 0.234 0.233  
Had tattooing (838, 2243) 2.99 0.053 0.067 0.05 0.008 0.010  
Had blood transfusion (838, 2242) 3.51 0.013 0.020 0.02 0.001 0.001  
Used any recreational drugs (831, 2227) 3.00 0.011 0.010 0.01 0.000 0.000  
Men who had sex with men (1128, 1815) 2.00 0.012 0.018 0.00 0.000 0.000  
Overall rural men 1.72     1.415 1.492 1.054
Urban women (Number not participated, number participated)        
More than one lifetime man sex partners (1058, 1941) 8.33 0.050 0.052 0.21 0.087 0.091  
Blood transfusion done (838, 2288) 2.08 0.074 0.099 0.01 0.002 0.002  
Overall urban women 1.90     0.089 0.093 1.045
Rural women (Number not participated, number participated)        
More than one lifetime man sex partners (826, 2270) 3.00 0.126 0.113 0.21 0.079 0.071  
Blood transfusion done (1113, 2015) 3.20 0.054 0.073 0.01 0.002 0.002  
Overall rural women 1.57     0.081 0.074 0.907
  1. aRelative impact of risk factors on HIV per unit population were published earlier from this study; these are based on computation of population impact number using the relative risk of HIV associated with that factor and the prevalence of that factor [26].
  2. bRelative impact on HIV estimation calculated by multiplying these three separately for those who participated and those who did not participate in the follow-up: HIV prevalence in those having that factor, proportion having this factor among those who participated/did not participate, and relative impact of that factor on HIV in a unit population.
  3. cRatio for each group (urban men, rural men, urban women, rural women) calculated by summing up the relative impact on HIV estimation for all factors in each group separately for those who participated and those who did not participate and divided the former number with the later. This ratio would give the over- or under-estimation of HIV in each group from those who participated in the follow-up based on the distribution of the risk factors in those who participated and those who did not.