Suchergebnisse

Political scientists are often called upon to estimate models in which the standard assumption that the data are conditionally independent can be called into question. I review the method of generalized estimating equations (GEE) for dealing with such correlated data. The GEE approach offers a number of advantages to researchers interested in modeling correlated data, including applicability to data in which the outcome variable takes on a wide range of forms. In addition, GEE models allow for substantial flexibility in specifying the correlation structure within cases & offer the potential for valuable substantive insights into the nature of that correlation. Moreover, GEE models are estimable with many currently available software packages, & the interpretation of model estimates is identical to that for commonly used models for uncorrelated data (eg, logit & probit). I discuss practical issues relating to the use of GEE models & illustrate their usefulness for analyzing correlated data through three applications in political science. 3 Tables, 3 Figures, 1 Appendix, 47 References. Adapted from the source document.

As applications of duration analysis have burgeoned in political science, scholars have become increasingly aware of the potential substantive importance of duration dependence: the extent to which the conditional hazards of the events of interest are rising or falling over time. Here I discuss the issue of duration dependence, focusing on the distinction between "spurious" dependence due to unobserved heterogeneity and "true" duration dependence due to state dependence in the process of interest. I present a simple extension of a commonly used parametric duration model—the Weibull model—which allows researchers to assess the influence of causal variables on the nature and extent of duration dependence in their data. I then illustrate the application of this "generalized Weibull" model using data on the duration of international alliances.

This article examines two alternative specifications for estimating event-count models in which the data-generating process results in a larger number of zero counts than would be expected under standard distributional assumptions. The author compares King's hurdle event count model and Greene's zero-inflated Poisson model, using data on congressional responses to Supreme Court decisions from 1979 to 1988. The author shows that each of these models is a special case of a more general dual regime data-generating process that results in extra-Poisson zero counts. Furthermore, because this data-generating process can produce overdispersion in its own right, these models are also shown to be related to variance function negative binomial specifications. The underlying correspondence between these models leads to similar results in estimating and interpreting them in practice.

Examines number of concurring and dissenting opinions in Supreme Court cases, to determine if leadership styles influence the tendency of individual justices to express opinions; 1800-1991; US.

A key assumption of nearly all widely used duration models is that the hazard ratios (ie, the conditional relative risks across substrata) are proportional to one another & that this proportionality is maintained over time. Estimation of proportional hazards models when hazards are nonproportional results in coefficient biases & decreased power of significance tests. Techniques for relaxing this assumption allow scholars to test whether the effects of covariates change over time & also permit a more nuanced understanding of the phenomenon being studied. We address the potential problems with incorrectly assuming proportionality, illustrate a number of tests for nonproportionality, & conclude with a discussion of how to accurately & efficiently estimate these models in the presence of nonproportional hazards. We investigate the proportionality assumption for Cox's semiparametric model in the context of the "liberal peace" debate, using data on international conflict in the postwar period. 3 Tables, 4 Figures, 2 Appendixes, 58 References. Adapted from the source document.