Department of Educational Studies, University of South Carolina, Columbia, SC 29208, USA.
Eval Rev. 2011 Dec;35(6):627-41. doi: 10.1177/0193841X12439195.
Covariate adjustment can increase the precision of estimates by removing unexplained variance from the error in randomized experiments, although chance covariate imbalance tends to counteract the improvement in precision. The author develops an easy measure to examine chance covariate imbalance in randomization by standardizing the average covariate difference between the treatment and control condition. The standardized covariate difference must not exceed an upper bound in order to gain precision in covariate adjusted analysis. The author then shows how to select an adequate sample size to mitigate chance covariate imbalance and improve precision in small pilot studies.
协变量调整可以通过从随机实验的误差中去除未解释的方差来提高估计的精度，尽管偶然的协变量不平衡往往会抵消精度的提高。作者通过标准化处理组和对照组之间的平均协变量差异，开发了一种简单的方法来检验随机分组中偶然的协变量不平衡。为了在协变量调整分析中获得精度，标准化的协变量差异不得超过上限。然后，作者展示了如何选择适当的样本量来减轻偶然的协变量不平衡，并在小型试点研究中提高精度。
