***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
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    ***EXAMPLES*** 
    Input: The propensity score is the probability of treatment assignment conditional on observed baseline characteristics . The propensity score allows one to design and analyze an observational ( nonr and omized ) study so that it mimics some of the particular characteristics of a r and omized controlled trial . In particular , the propensity score is a balancing score : conditional on the propensity score , the distribution of observed baseline covariates will be similar between treated and untreated subjects . I describe 4 different propensity score methods : matching on the propensity score , stratification on the propensity score , inverse probability of treatment weighting using the propensity score  , and covariate adjustment using the propensity score . I describe balance diagnostics for examining whether the propensity score model has been adequately specified . Furthermore , I discuss differences between regression-based methods and propensity score-based methods for the analysis of observational data . I describe different causal average treatment effects and their relationship with propensity score analyses Background : Intraoperative awareness with explicit recall occurs in approximately 0.15 % of all surgical cases . Efficacy trials based on the Bispectral Index ® ( BIS ) monitor ( Covidien , Boulder , CO ) and anesthetic concentrations have focused on high-risk patients , but there are no effectiveness data applicable to an unselected surgical population . Methods : We conducted a r and omized controlled trial of unselected surgical patients at three hospitals of a tertiary academic medical center . Surgical cases were r and omized to alerting algorithms based on either BIS values or anesthetic concentrations . The primary outcome was the incidence of definite intraoperative awareness ; prespecified secondary outcomes included postanesthetic recovery variables . Results : The study was terminated because of futility . At interim analysis the incidence of definite awareness was 0.12 % ( 11/9,376 ) ( 95 % CI : 0.07–0.21 % ) in the anesthetic concentration group and 0.08 % ( 8/9,460 ) ( 95 % CI : 0.04–0.16 % ) in the BIS group ( P = 0.48 ) . There was no significant difference between the two groups in terms of meeting criteria for recovery room discharge or incidence of nausea and vomiting . By post hoc secondary analysis , the BIS protocol was associated with a 4.7-fold reduction in definite or possible awareness events compared with a cohort receiving no intervention ( P = 0.001 ; 95 % CI : 1.7–13.1 ) . Conclusion : This negative trial could not detect a difference in the incidence of definite awareness or recovery variables between monitoring protocol s based on either BIS values or anesthetic concentration . By post hoc analysis , a protocol based on BIS monitoring reduced the incidence of definite or possible intraoperative awareness compared with routine care Background : Hospitals are increasingly required to publicly report outcomes , yet performance is best interpreted in the context of population and procedural risk . We sought to develop a risk-adjustment method using administrative cl aims data to assess both national-level and hospital-specific performance . Methods : A total of 35,179,507 patient stay records from 2001–2006 Medicare Provider Analysis and Review ( MEDPAR ) files were r and omly divided into development and validation sets . Risk stratification indices ( RSIs ) for length of stay and mortality endpoints were derived from aggregate risk associated with individual diagnostic and procedure codes . Performance of RSIs were tested prospect ively on the validation data base , as well as a single institution registry of 103,324 adult surgical patients , and compared with the Charlson comorbidity index , which was design ed to predict 1-yr mortality . The primary outcome was the C statistic indicating the discriminatory power of alternative risk-adjustment methods for prediction of outcome measures . Results : A single risk-stratification model predicted 30-day and 1-yr postdischarge mortality ; separate risk-stratification models predicted length of stay and in-hospital mortality . The RSIs performed well on the national data set ( C statistics for median length of stay and 30-day mortality were 0.86 and 0.84 ) . They performed significantly better than the Charlson comorbidity index on the Clevel and Clinic registry for all outcomes . The C statistics for the RSIs and Charlson comorbidity index were 0.89 versus 0.60 for median length of stay , 0.98 versus 0.65 for in-hospital mortality , 0.85 versus 0.76 for 30-day mortality , and 0.83 versus 0.77 for 1-yr mortality . Addition of demographic information only slightly improved performance of the RSI . Conclusion : RSI is a broadly applicable and robust system for assessing hospital length of stay and mortality for groups of surgical patients based solely on administrative data BACKGROUND : A correlation between deep anesthesia ( defined as time with Bispectral Index ( BIS ) < 45 ; TBIS < 45 ) and death within 1 yr after surgery has previously been reported . In order to confirm or refute these findings , we evaluated TBIS < 45 as an independent risk factor for death within 1 and 2 yr after surgery and also the impact of malignancy , the predominant cause of death in the previous report . METHODS : Mortality within 2 yr after surgery , causes of death and the occurrence of malignant disease at the time of surgery were identified in a cohort of 4087 BIS-monitored patients . Statistically significant univariate predictors of mortality were identified . In order to allow for comparison with previous data , the following multivariate analysis was first done without , and thereafter with , preexisting malignancy status , the predominant cause of death . RESULTS : One-hundred-seventy-four ( 4.3 % ) patients died within 1 yr and another 92 during the second year ( totaling 6.5 % in 2 yr ) . TBIS < 45 was a significant predictor of 1- and 2-yr mortality when preexisting malignant disease was not among the co-variates ( hazard ratio [ HR ] 1.13 [ 1.01–1.27 ] and 1.18 [ 1.08–1.29 ] , respectively ) . Further exploration confined the significant relation between postoperative mortality and TBIS < 45 to patients with preexisting malignant diagnoses associated with extensive surgery and less favorable prognosis . The most powerful predictors of 2-yr mortality in the model , including preexisting malignancy , were ASA physical score class IV ( HR 19.3 [ 7.31–51.1 ] ) , age > 80 yr ( HR 2.93 [ 1.79–4.79 ] ) , and preexisting malignancy associated with less favorable prognosis ( HR 9.30 [6.60–13.1]).When the initial multivariate regression was repeated using preexisting malignancy status among the co-variates in the model , the previously significant relation between 1 , and 2-yr mortality and TBIS < 45 did not reach statistical significance . CONCLUSION : Using a similar set of co-variates as in previous work , we confirmed the statistical relation between 1-yr mortality and TBIS < 45 , and we extended this observation to 2-yr mortality . However , this relation is sensitive to the selection of co-variates in the statistical model , and a r and omized study is required to demonstrate that there really is a causal impact from and TBIS < 45 on postoperative mortality and , if it does , the effect is probably very weak in comparison with co-morbidity as assessed by ASA physical score , the preexisting malignancy status at surgery and age BACKGROUND Unintended intraoperative awareness , which occurs when general anesthesia is not achieved or maintained , affects up to 1 % of patients at high risk for this complication . We tested the hypothesis that a protocol incorporating the electroencephalogram-derived bispectral index ( BIS ) is superior to a protocol incorporating st and ard monitoring of end-tidal anesthetic-agent concentration ( ETAC ) for the prevention of awareness . METHODS We conducted a prospect i ve , r and omized , evaluator-blinded trial at three medical centers . We r and omly assigned 6041 patients at high risk for awareness to BIS-guided anesthesia  ( with an audible alert if the BIS value was < 40 or > 60 , on a scale of 0 to 100 , with 0 indicating the suppression of detectable brain electrical activity and 100 indicating the awake state ) or ETAC-guided anesthesia ( with an audible alert if the ETAC was < 0.7 or > 1.3 minimum alveolar concentration ) . In addition to audible alerts , the protocol s included structured education and checklists . Superiority of the BIS protocol was assessed with the use of a one-sided Fisher 's exact test . RESULTS A total of 7 of 2861 patients ( 0.24 % ) in the BIS group , as compared with 2 of 2852 ( 0.07 % ) in the ETAC group , who were interviewed postoperatively had definite intraoperative awareness ( a difference of 0.17 percentage points ; 95 % confidence interval [ CI ] , -0.03 to 0.38 ; P=0.98 ) . Thus , the superiority of the BIS protocol was not demonstrated . A total of 19 cases of definite or possible intraoperative awareness ( 0.66 % ) occurred in the BIS group , as compared with 8 ( 0.28 % ) in the ETAC group ( a difference of 0.38 percentage points ; 95 % CI , 0.03 to 0.74 ; P=0.99 ) , with the superiority of the BIS protocol again not demonstrated . There was no difference between the groups with respect to the amount of anesthesia administered or the rate of major postoperative adverse outcomes . CONCLUSIONS The superiority of the BIS protocol was not established ; contrary to expectations , fewer patients in the ETAC group than in the BIS group experienced awareness . ( Funded by the Foundation for Anesthesia Education and Research and others ; BAG-RECALL Clinical Trials.gov number , NCT00682825 . ) Little is known about the effect of anesthetic management on long-term outcomes . We design ed a prospect i ve observational study of adult patients undergoing major noncardiac surgery with general anesthesia  to determine if mortality in the first year after surgery is associated with demographic , preoperative clinical , surgical , or intraoperative variables .  One-year mortality was 5.5 % in all patients ( n = 1064 ) and 10.3 % in patients ≥65 yr old ( n = 243 ) . Multivariate Cox Proportional Hazards modeling identified three variables as significant independent predictors of mortality : patient comorbidity ( relative risk , 16.116 ; P < 0.0001 ) , cumulative deep hypnotic time ( Bispectral Index ® < 45 ) ( relative risk = 1.244/h ; P = 0.0121 ) and intraoperative systolic hypotension ( relative risk = 1.036/min ; P = 0.0125 ) . Death during the first year after surgery is primarily associated with the natural history of preexisting conditions . However , cumulative deep hypnotic time and intraoperative hypotension were also significant , independent predictors of increased mortality . These associations suggest that intraoperative anesthetic management may affect outcomes over longer time periods than previously appreciated BACKGROUND : When anesthesia is titrated using bispectral index ( BIS ) monitoring , patients generally receive lower doses of hypnotic drugs . Intraoperative hypotension and organ toxicity might be avoided if lower doses of anesthetics are administered , but whether this translates into a reduction in serious morbidity or mortality remains controversial . The B-Aware Trial r and omly allocated 2463 patients at high risk of awareness to BIS-guided anesthesia or routine care .  We tested the hypothesis that the risks of death , myocardial infa rct ion ( MI ) , and stroke would be lower in patients allocated to BIS-guided management than in those allocated to routine care . METHODS : The medical records of all patients who had not died within 30 days of surgery were review ed . The date and cause of death and occurrence of MI or stroke were recorded . A telephone interview was then conducted with all surviving patients . The primary end point of the study was survival . RESULTS : The median follow-up time was 4.1 ( range : 0–6.5 ) years . Five hundred forty-eight patients ( 22.2 % ) had died since the index surgery , 220 patients ( 8.9 % ) had an MI , and 115 patients ( 4.7 % ) had a stroke . The risk of death in BIS patients was not significantly different than in routine care patients ( hazard ratio = 0.86 [ 95 % confidence interval { CI } : 0.72–1.01 ] ; P = 0.07 ) . However , propensity score analysis indicated that the hazard ratio for death in patients who recorded BIS values < 40 for > 5 min compared with other BIS-monitored patients was 1.41 ( 95 % CI : 1.02–1.95 ; P = 0.039 ) . In addition , the odds ratios for MI in patients who recorded BIS values < 40 for > 5 min compared with other BIS-monitored patients was 1.94 ( 95 % CI : 1.12–3.35 ; P = 0.02 ) and the odds ratio for stroke was 3.23 ( 95 % CI : 1.29–8.07 ; P = 0.01 ) . CONCLUSIONS : Monitoring with BIS 
    Output:
    
Conclusion The present study demonstrated that patients exposed to the triple low state exhibit higher 30-day mortality rates than those not exposed to the triple low state