group, ranking based on adjusted measures varied for 13 hospitals: seven differed 1 rank, five differed 2–3 ranks, one differed 7 ranks. Spearman’s correlation coefficients between crude NTCS and adjusted primary CD rates and between crude and adjusted NTCS CD rates were r = 0.81 and r = 0.97. In the year 2009, in the Tuscany Region, of the 29,438 deliveries, 26,851 were deliveries without a previous CD across 26 hospitals and 12,433 met the NTCS inclusion criteria across 25 hospitals One hospital was excluded in the analyses of the NTCS study group because of incomplete data on parity. In women without a previous CD, the reference category included one hospital with a CD rate of 9.2%. After adjusting for clinical and socio-demographic variables, 10 hospitals showed a difference in RR greater than 10% and twenty-two hospitals changed their rank: five hospitals differed 1 rank, fourteen differed 2–3 ranks, three differed 4–10 ranks. In the NTCS group the reference category included three hospitals with an average rate of 11.1%. After adjusting for clinical and socio-demographic variables, a difference in adjusted RR higher than 10% was found in 5 hospitals, and all the adjusted RR were lower than the crude RR. Ranking based on adjusted measures varied for 14 hospitals: nine differed 1 rank, five differed 2–3 ranks. Spearman’s correlation coefficient between crude and adjusted primary CD rates was r = 0.92 and between crude and adjusted NTCS CD rates was r = 0.98. In both regions, a sensitivity analysis was carried out to explore the role of cephalopelvic disproportion as potential confounder. Hospital adjusted RRs (for primary and NTCS CD) were calculated after excluding this variable from the model. No substantial differences were observed between adjusted RRs obtained with and without this variable (data available on request). In a secondary sensitivity analysis we excluded all deliveries with a clinical condition resulting in appropriate CD (genital HSV, HIV, cord prolapse, malpresentation) and calculated adjusted RR in each cohort of the two regions. In the cohort of primary CD, a .10% difference between crude and adjusted RR was found in 15 hospitals in Emilia-Romagna and 15 in Tuscany. In the NTCS group, a .10% difference between crude and adjusted RR was found in 4 hospital in Emilia-Romagna and 8 in Tuscany. Identification of outliers In the present study, interhospital comparison is based on the use of a reference group of hospitals with the best performance. Outliers are therefore those with the highest difference from the reference group, after adjusting for case mix. Alternative methods for interhospital comparison using the regional mean as the reference group allow identification of outliers with adjusted CD rates lower or higher than the regional mean. For example, using the funnel plot as the graphical representation of adjusted CD rates vs. the number of deliveries, we found that, in Tuscany, 5 hospitals had adjusted CD rates exceeding the upper level of the 99.8% confidence interval of the mean. Of note, in the NTCS group, the outlier hospitals with significantly higher unadjusted CD rates than the regional mean (Figure 1) were the same as those in the primary CD group. Discussion Our results indicate substantial inter-hospital variations in both primary and NTCS CD rates. NTCS CD contributed more than one third of the overall CD rate, and more than one half of primary CD, in line with other studies [4,9,14,22,23]. The main strength of our study consists in the relatively large number of deliveries from 50 hospitals of two different regions. Inter-Hospital Comparison of Caesarean Delivery Rates We found that the use of risk adjustment procedures had a different importance when comparing hospitals in terms of primary or NTCS CD rates. When the focus was on primary CD rates, adjustment for clinical and socio-demographic factors affected the RRs in several hospitals and led to a substantial variation in their ranking. On the contrary, in the NTCS group, risk adjustment led to a lower change in RRs and more limited variation in the hospital ranking. Morevoer, crude NTCS CD rates highly correlated with adjusted NTCS CD rates. These findings were consistent between the two Italian regions studied. In order to explain the modest difference between crude and adjusted RR in NTCS compared to primary CD, it must be recalled that the NTCS groups is homogeneous for gestational age, presentation, singleton birth and nulliparity by definition. Therefore our results suggest that the NTCS cohort excludes frequent conditions that may be unevenly distributed across hospitals. All other factors we controlled for in this cohort, including mother’s comorbidities or demographic variables and other less frequent obstetric conditions did not confound substantially the results. Sensitivity analyses excluding from the primary and NTCS cohorts factors for which the CD is appropriate (HIV, malpre- sentation, cord prolapse and genital herpes) did not reduce interhospital variability and the discrepancy between crude and adjusted rates. Lastly, we found that outlier hospitals with high CD rates can be consistently identified using two methods for interhospital comparison that use best performing hospitals or the regional mean as the reference group. Furthermore, outlier hospitals in the primary CD and NTCS cohort are the same. Inter-hospital variation of CD rates persisted after adjustment for clinical and socio-demographic variables in all women without a previous CD and in the NCTS group, suggesting that variablility should be ascribed to other unexplored clinical or non medical factors, including organization in the birth units, staff attitudes, cultural backgrounds and women’s choice [24–26]. Few studies have addressed the relevance of applying risk adjustment procedures to compare NTCS CD rates across hospitals. Main et al [4] reported that after controlling for age, some hospitals showed small differences in CD rates, but 5 out of 20 with age-skewed populations had a 2.5–5% reduction in CD rates. They concluded that age-adjusted NTCS CD rates are a promising quality measure. Coonrod et al. [14] using NTCS CD rates, reported that 31/40 hospitals retained their original ranking before and after adjustment for clinical factors, but 4 changed their status from an outlier to an average-risk hospital. These authors argued that hospital quality assessment programs may require risk adjusted NTCS CD rates [14]. Our results should be interpreted keeping in mind some limitations. First, we used administrative databases, consistent with studies monitoring CD rates for quality of care assessment. Multiple issues regarding the validity of administrative data remain largely unexplored [27]. Problems in accuracy, completeness and quality might differ from hospital to hospital, errors in coding may occur and omissions of ICD codes identifying risk factors may be more likely in the group without a CD. However, in Emilia Romagna and Tuscany Region, the administrative databases proved to have a high degree of completeness and quality and have already been used in studies using the same data sources [28]. Second, other risk factors for CD such as body mass index and gestational weight gain [29] could not be included in the risk adjustment model because information on height and weight is not recorded in our databases. Nevertheless, to date administrative databases continue to be the most viable solution to study PLoS ONE | www.plosone.org 3 November 2011 | Volume 6 | Issue 11 | e28060