To control for potential confounding effects of geography , 16 temporal trends in outcomes , and hospital characteristics , we used a concurrent control group for comparison . This group consisted of all hospital admissions with an intensive care unit stay from hospitals located across the other 11 states in the US Midwest region ( as defined by the US Census Bureau ) , with hospital selection based on a random sampling methodology stratified by hospital bed size and teaching status Unadjusted and adjusted population level data on hospital mortality and average length of stay in Michigan hospitals and comparison hospitals ( 364 hospitals ) . Using this sampling methodology , we initially selected 64 % ( n=631 ) of hospitals in the Midwest region for the comparison group , including all large teaching , large non - teaching , and small teaching hospitals in the region . After exclusions , the final comparison group represented 37 % ( n=364 ) of Midwest hospitals . The sample size calculation was conservatively based on the number of hospitals , rather than hospital admissions , because the analysis accounted for clustering of admissions within hospitals . The sample size calculation assumed a power of 0.80 , a two tailed α of 0.05 , detection of a difference in reduction in length of stay between study and comparison group hospitals of 0.1 days ( group standard deviation 0.3 ) , and a fixed study group size of 90 hospitals ( an initial conservative estimate of the number of Michigan hospitals during the study period ) , resulting in a 4:1 ratio of comparison to study group hospitals .