Multiple Imputation For Analysis Research Articles

A Two-Step Multiple Imputation for Analysis of Repeated Measures With Left-Censored and Missing Data

Left-censored data can occur in many clinical trials in which laboratory measures are obtained with a quantitative analysis assay. Some common examples are longitudinal viral load measures in HIV drug trials and antibody measures in vaccine studies. Conventionally, an analysis of covariance model or constraint longitudinal data analysis model may be applied in the analysis of log-transformed responses, with the left-censored values replaced with the detection limit or half of the limit. When the proportion of left censoring is moderate to large, this single ad hoc imputation method may lead to bias in parameter estimates and inappropriate alpha levels for statistical tests. In this article, we propose a two-step multiple imputation (MI) approach to deal with the censored and missing data. The proposed two-step MI can be implemented using common software such as SAS procedures. Simulations are conducted under various scenarios to evaluate the performance of this method compared with the conventional methods. Applications to real clinical trials are also given to illustrate the potential impact and benefit of the proposed method. Supplementary materials for this article are available online.

Application of Multiple Imputation in Analysis of Data from Clinical Trials with Treatment Related Dropouts

Many longitudinal clinical studies suffer from nonignorable dropouts. Ramakrishnan and Wang (2005) proposed a mixed effects analysis treating missing data as missing due to truncation (MDT), estimated the parameters under a multivariate truncated normal model, and suggested an adjustment to the degrees of freedom to account for the implicit estimation of the missing data. We propose a multiple imputation (MI) in conjunction with the MDT method as an alternative to accurately accommodate the uncertainty introduced by imputation. The data used in Ramakrishnan and Wang (2005) is considered for illustration. A comparison of various methods using a simulation study is presented.