POCD is assessed with a standard cognitive test battery by staff trained by a board-certified neuropsychologist; the full test battery takes approximately 30 to 60 minutes to complete. Individual test scores are combined by factor analysis into four cognitive domains:verbal memory, executive function, attention and concentration, and visual memory.The mean of these domain scores yields the Continuous Cognitive Index(CCI), a sensitive score used to quantify overall cognitive function. CCI change from before to after surgery thus quantifies the degree of learning or cognitive decline.POCD as a dichotomous outcome is defined as an SD drop of 1 or more from before to 6 weeks after surgery in one or more cognitive domain.