PubMed · 10819686
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R H Green, I D Pavord. 2000. Asthma guidelines.. https://doi.org/10.1136/thorax.55.5.439b
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This paper considers the mixture model methodology for handling non-ignorable drop-outs in longitudinal studies with continuous outcomes. Recently, Hogan and Laird have developed a mixture model for non-ignorable drop-outs which is a standard linear mixed effects model except that the parameters which characterize change over time depend also upon time of drop-out. That is, the mean response is linear in time, other covariates and drop-out time, and their interactions. One of the key attractions of the mixture modelling approach to drop-outs is that it is relatively easy to explore the sensitivity of results to model specification. However, the main drawback of mixture models is that the parameters that are ordinarily of interest are not immediately available, but require marginalization of the distribution of outcome over drop-out times. Furthermore, although a linear model is assumed for the conditional mean of the outcome vector given time of drop out, after marginalization, the unconditional mean of the outcome vector is not, in general, linear in the regression parameters. As a result, it is not possible to parsimoniously describe the effects of covariates on the marginal distribution of the outcome in terms of regression coefficients. The need to explicitly average over the distribution of the drop-out times and the absence of regression coefficients that describe the effects of covariates on the outcome are two unappealing features of the mixture modelling approach. In this paper we describe a particular parameterization of the general linear mixture model that circumvents both of these problems.
The prevalence of asthma in children has increased 160 percent since 1980, and the disease currently affects nearly 5 million children in the United States. The National Asthma Education and Prevention Program provides guidelines for improved asthma care. The goals of this program are to limit the frequency, severity and costliness of asthma exacerbations through extensive education of physicians, children and caregivers. The four components of asthma management include regular assessment and monitoring, control of factors that contribute to or aggravate symptoms, pharmacologic therapy and education of children and their caregivers. The guidelines recommend a stepwise approach to pharmacologic treatment, starting with aggressive therapy to achieve control and followed by a "step down" to the minimal therapy that will maintain control. Quick relief of symptoms can be achieved preferentially by the use of short-acting beta2 agonists. Medications for long-term control should be considered for use in children with persistent symptoms. Inhaled corticosteroids are the most potent long-term anti-inflammatory medications. Other options include long-acting beta2 agonists, cromolyn sodium and nedocromil, antileukotriene agents and theophylline. All have advantages and disadvantages in individual situations.