在文献中读到这样一句话，Patient demographics, comorbidities, perioperative characteristics and clinical outcomes were summarized with mean (standard deviation) and n (%), and compared using hierarchical regression, clustering patients within

sites.后面有图表有adjusted P-value for site clusters这一列，请问这一列该如何理解，只知道和聚类有关，但是是什么意思呢？