There is one fairly common circumstance in which both convergence problems and the Hauck-Donner phenomenon (and trouble with \sfn{step}) can occur. This is when the fitted probabilities are extremely close to zero or one. Consider a medical diagnosis problem with thousands of cases and around fifty binary explanatory variables (which may arise from coding fewer categorical factors); one of these indicators is rarely true but always indicates that the disease is present. Then the fitted probabilities of cases with that indicator should be one, which can only be achieved by taking \hat\beta_i = \infty. The result from \sfn{glm} will be warnings and an estimated coefficient of around +/- 10 [and an insignificant t value].

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There is one fairly common circumstance in which both convergence problems and the Hauck-Donner phenomenon (and trouble with \sfn{step}) can occur. This is when the fitted probabilities are extremely close to zero or one. Consider a medical diagnosis problem with thousands of cases and around fifty binary explanatory variables (which may arise from coding fewer categorical factors); one of these indicators is rarely true but always indicates that the disease is present. Then the fitted probabilities of cases with that indicator should be one, which can only be achieved by taking \hat\beta_i = \infty. The result from \sfn{glm} will be warnings and an estimated coefficient of around +/- 10 [and an insignificant t value].

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Il s'agit de prédire la valeur d'une variables qualitative, i.e., de mettre les individus dans des classes. (Par exemple : aide au diagnostic médical, reconnaissance des mauvais payeurs par une banque, etc.) On cherche des "fonctions linéaires discirminantes (des combinaisons linéaires dea variables, qui maximisent la variance interclasse et minimisent la variance intraclasse)

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Il s'agit de prédire la valeur d'une variables qualitative, i.e., de mettre les individus dans des classes. (Par exemple : aide au diagnostic médical, reconnaissance des mauvais payeurs par une banque, etc.) On cherche des "fonctions linéaires discirminantes (des combinaisons linéaires dea variables, qui maximisent la variance interclasse et minimisent la variance intraclasse)

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To expand a little, if |t| is small it can EITHER mean than the Taylor expansion works and hence the likelihood ratio statistic is small OR that |\hat\beta_i| is very large, the approximation is poor and the likelihood ratio statistic is large. (I was using `significant' as meaning practically important.) But we can only tell if |\hat\beta_i| is large by looking at the curvature at \beta_i=0, not at |\hat\beta_i|. This really does happen: from later on in V&R2:

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There is a little-known phenomenon for binomial GLMs that was pointed out by Hauck & Donner (1977: JASA 72:851-3). The standard errors and t values derive from the Wald approximation to the log-likelihood, obtained by expanding the log-likelihood in a second-order Taylor expansion at the maximum likelihood estimates. If there are some \hat\beta_i which are large, the curvature of the log-likelihood at \hat{\vec{\beta}} can be much less than near \beta_i = 0, and so the Wald approximation underestimates the change in log-likelihood on setting \beta_i = 0. This happens in such a way that as |\hat\beta_i| \to \infty, the t statistic tends to zero. Thus highly significant coefficients according to the likelihood ratio test may have non-significant t ratios.

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