Centre for Research in Environmental Epidemiology (CREAL), Barcelona, Spain; IMIM (Hospital del Mar Medical Research Institute), Barcelona, Spain; CIBER Epidemiología y Salud Pública (CIBERESP), Barcelona, Spain.
Stat Med. 2013 Nov 30;32(27):4748-62. doi: 10.1002/sim.5870. Epub 2013 Jun 5.
In the context of observational longitudinal studies, we explored the values of the number of participants and the number of repeated measurements that maximize the power to detect the hypothesized effect, given the total cost of the study. We considered two different models, one that assumes a transient effect of exposure and one that assumes a cumulative effect. Results were derived for a continuous response variable, whose covariance structure was assumed to be damped exponential, and a binary time-varying exposure. Under certain assumptions, we derived simple formulas for the approximate solution to the problem in the particular case in which the response covariance structure is assumed to be compound symmetry. Results showed the importance of the exposure intraclass correlation in determining the optimal combination of the number of participants and the number of repeated measurements, and therefore the optimized power. Thus, incorrectly assuming a time-invariant exposure leads to inefficient designs. We also analyzed the sensitivity of results to dropout, mis-specification of the response correlation structure, allowing a time-varying exposure prevalence and potential confounding impact. We illustrated some of these results in a real study. In addition, we provide software to perform all the calculations required to explore the combination of the number of participants and the number of repeated measurements.
在观察性纵向研究的背景下，我们探讨了在给定研究总成本的情况下，最大化检测假设效果的能力的参与者数量和重复测量次数的价值。我们考虑了两种不同的模型，一种假设暴露的瞬时效应，另一种假设累积效应。结果是针对连续响应变量推导的，假设其协方差结构为阻尼指数，并考虑了二分类时变暴露。在某些假设下，我们推导出了在响应协方差结构假设为复合对称的特殊情况下，该问题近似解的简单公式。结果表明，暴露内相关系数在确定参与者数量和重复测量次数的最佳组合方面具有重要意义，因此也决定了优化的功效。因此，错误地假设暴露是时不变的会导致设计效率低下。我们还分析了结果对辍学、响应相关结构的错误指定、允许时变暴露流行率和潜在混杂影响的敏感性。我们在一项真实研究中说明了其中的一些结果。此外，我们提供了软件来执行所有计算，以探索参与者数量和重复测量次数的组合。
