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Table 5 Comparison of HIV seroprevalence estimates based on standard probit models and models accounting for sample selection under various scenarios (Zewditu Memorial Hospital, Addis Ababa, 2003–04)

From: Implications of the HIV testing protocol for refusal bias in seroprevalence surveys

  Scenario
  Test of Heckman modela Post-test counseling is required Post-test counseling is optional
E(HIV% – Probit) 17.7
(16.4 – 19.1)
17.8
(16.6–19.1)
21.4
(20.2–22.7)
E(HIV% – Heckman) 23.1
(21.7 – 24.4)
23.4
(22.1–24.7)
23.7
(22.4–25.0)
Observed HIV% 22.2
(19.9 – 24.4)
unknown unknown
Sample Consent groups A and B All consent groups All consent groups
Assumption HIV status in consent group B is unobserved HIV status in consent groups B and C is unobserved HIV status in consent group C is unobserved
LR test H0:ρ = 0 p < .01 p < .01 p = .07
  1. Notes: 95%- CI are reported between brackets. Using dummies for admission diagnosis rather than the likelihood of infection in these regressions hardly changes the estimated prevalence rates though one of the selection models did not converge.
  2. a In the first column, we assume that HIV status in consent group B is unknown, and compare the ordinary Probit and Heckman selection model estimate with the true or observed value of HIV prevalence.