含基线功能协变量的功能联合模型：关联老年女性的久坐积累模式与身体功能及死亡率
A functional joint model with baseline functional covariates: linking sitting accumulation patterns to physical function and mortality among older women
AI总结：
本研究针对老年女性，提出含基线功能协变量的功能联合模型，分析OPACH研究中6489名女性的久坐积累模式与身体功能、死亡率的关联，验证了该模型的有效性并揭示了相关剂量-反应关系。
AI中文摘要：
在大规模流行病学研究中，研究者常关注以轨迹或函数形式存在的暴露因素与纵向结局、时间-事件结局之间的联合关系。本研究的动机来源于客观身体活动与心血管健康（OPACH）研究，该研究采集了6489名老年女性的加速度计测量身体活动数据，其中一项科学目标是明确久坐行为的积累模式及其与纵向结局身体功能、时间-事件结局死亡率的关联。我们提出一种新方法：首先将原始加速度计数据转换为每日久坐发作积累轮廓，将其作为功能协变量，随后开发纳入基线功能协变量的纵向与时间-事件结局功能联合模型。纵向过程采用功能数据方法建模，通过功能主成分得分与生存过程关联；两个子模型均包含可解释的线性标量-函数回归系数，以捕捉不同发作时长下久坐积累与健康结局之间灵活的剂量-反应关联。估计通过带惩罚样条近似的高效期望最大化（EM）算法实现。模拟研究表明该方法参数估计准确、模型选择可靠；将其应用于OPACH数据，揭示了久坐发作时长、身体功能与死亡率之间灵活且可解释的剂量-反应关系。
英文摘要：
In large-scale epidemiological studies, it is often of interest to investigate joint relationships between longitudinal and time-to-event outcomes with exposures that are trajectories or functions. Our motivation study is the Objective Physical Activity and Cardiovascular Health (OPACH) Study, which collected accelerometry-measured physical activity in 6,489 older women. One of the scientific aims is to understand sedentary behavior accumulation patterns and its association with physical function (longitudinal) and mortality (time-to-event). We propose a novel approach that first converts raw accelerometry data into daily sitting bout accumulation profiles, which are treated as functional covariates, and then develop a functional joint model for longitudinal and time-to-event outcomes that incorporates a baseline functional covariate. The longitudinal process is modeled using functional data methods and linked to the survival process through functional principal component scores. Both sub-models include interpretable linear scalar-on-function regression coefficients to capture flexible dose-response associations between sitting accumulated across varying bout durations and health outcomes. Estimation is carried out via an efficient expectation-maximization (EM) algorithm with penalized spline approximations. Simulation studies demonstrate accurate parameter estimation and reliable model selection. Application to the OPACH data reveals flexible and interpretable dose-response relationships between sitting bout durations, physical function, and mortality.
