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Unformatted text preview: substantively from the nonlinear biprobit specification in column 4, Table 8; however, the magnitudes of the coefficients and statistical significance do vary slightly. In particular, while the coefficient on GotResults does not differ greatly among the HIV-negatives (Table 8, panel B), among HIV-positives the coefficient in the nonlinear case is statistically significant at the 99 percent level (Table 8, panel A). The coefficient is also greater in the bivariate probit specification than in the linear model, suggesting an even larger effect of learning HIV-positive results on the likelihood of purchasing condoms. On the other hand, the difference between the two specifications may be due in part to the small sample size of HIV-positives. Given the difficulties with this estimation, including in particular the fact that it is necessary to separate the samples when doing the biprobit estimation, the linear IV specification in Table 7 remains the preferred estimation model.18 17 The bivariate probit estimates for HIV-positive and HIV-negative individuals were calculated by iterating over maximization techniques (Newton-Raphson, Berndt-Hall-Hall-Hausman, Davidon-Fletcher-Powell, and BroydenFletcher-Goldfarb-Shanno algorithms). Given the low sample size of HIV-positives, convergence to a reasonable estimate of r is difficult. I therefore constrain r among HIV-positives to be equal to that among HIV-negatives (0.218). 18 See also Angrist (2001) who argues in favor of conventional 2SLS estimation of nonlinear models. 1852 DEcEMBER 2008 THE AMERIcAN EcONOMIc REVIEW Table 8—Probit Estimates: Reactions to Learning HIV Results 0.427*** 1 0.149 2 0.038 1 0.115 2 0.001 1 0.040 2 0.000 1 0.0012 0.025 1 0.074 2 OLS (1) panel A: HIV positive Got results Male Age Age2 Simulated average distance Rumphi Constant Sample size R2 Mean panel B: HIV negative Got results Male Age Age2 Simulated average distance Rumphi Constant Sample size R2 Mean 20.084 1 0.179 2 0.055 1 0.823 2 52 0.21 0.38 20.023 1 0.025 2 0...
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This document was uploaded on 01/28/2014.

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