Although positive trends in patient care on ACE units have been shown, there are some limitations to the research. The current evidence is greatly limited by the use of 1 data set for multiple reports. All 4 of the RCTs took place in Ohio, where ACE was first developed.19, 20, 21, 22, 23 and 24 Three of the 4 reports relied on the same data set collected at the University Hospitals of Cleveland from November 1990 through March 1992.19, 20 and 24 This limits the generalizability of the studies to examine the effectiveness of ACE outside of this geographic area and this time period. The available literature is less diverse when different analyses of the same data are used, instead of replicating the ACE unit in different time periods or environments. Also, the limitations of the data collection are carried into all 3 studies. For example, the ACE unit had dedicated nursing and housekeeping staff, but the attending and resident physicians cared for by the ACE unit intervention group as well as the usual care control group; this creates the potential for contamination of the control group. Currently, the available data on ACE units is limited by the use of the same data for multiple reports.