The study cohort includes women who delivered in public and private accredited Emilia Romagna birth units from January 1, 2007 to June 30, 2009 and in Tuscany birth units from January 2009 to December 2009. Hospital discharge records were identified using Disease Related Groups (DRGs) 370-375 or ICD-9 CM codes in primary or secondary diagnosis (V27xx or 640.xy-676.xy, where x = 0,…,9 and y = 1 or 2) or intervention codes (72.x, 73.2, 73.5, 73.6, 73.8, 73.9, 74.0, 74.1, 74.2, 74.4, 74.99). We excluded the following: 1) mothers under 11 and over 55 years of age, 2) mothers discharged from hospitals without an operating room or small hospitals (,150 deliveries per year), 3) mothers having hospital discharge records including intrauterine death (ICD-9 CM code 656.4) and still births (ICD-9 CM code V27.1, V27.4, V27.7). We used birth certificates to identify parity, gestational age, plurality and presentation. We then selected two groups: N Women without a previous CD (ICD-9 CM diagnosis code 654.2x). N NTCS – nulliparous, term, cephalic, singleton deliveries (defined by birth certificate information). Maternal age, educational level, citizenship and marital status were retrieved from birth certificates. Information on maternal and foetal clinical risk factors was retrieved from hospital discharge records (index and previous hospitalisations) and/or birth certificates. This included: HIV, diabetes, hypertension, thyroid diseases, other severe comorbidities, genital herpes, substance abuse, eclampsia or pre-eclampsia, placenta praevia or abruption or ante partum hemorrhage, cephalopelvic disproportion, RH isoimmunisation, polihydramnios, oligohydramnios, premature rupture of membranes of the amnios, other problems of the amnios, cord prolapse, abortion threads, in vitro fertilization, or supervision of high risk pregnancy, intra-uterine growth retarda- tion, foetal weight, foetal malformation. We also examined pregnancy length, multiple births and presentation other than vertex as additional potential independent risk factors for primary CD, but not for NTCS. We excluded foetal distress and uterine dystocia as potential risk factors, because these might be a reason for an ex-post justification of the CD [16–18]. The study was carried out in compliance with the Italian law on privacy (Art. 20–21, DL 196/2003) and the regulations of the Regional Health Authorities of Emilia-Romagna and Tuscany Regions on data management. Data were anonymized at the regional statistical office where each patient was assigned a unique identifier that is the same for all administrative databases. This identifier does not allow to trace the patient’s identity and other sensitive data. When anonymized administrative data are used to inform health care planning activities, the study is exempt from notification to the Ethics Committee and no specific written consent is needed to use patient information stored in the hospital databases. Statistical analysis Primary and NTCS CD rates and their coefficient of variation (CV) by hospital were estimated. The relative risk of CD for each hospital was estimated in women without a previous CD and in the NTCS group using Poisson regression models, to control for demographic and clinical confounders [19]. Demographic and clinical risk factors of CD to be included in the risk adjustment models were selected using a backward stepwise selection procedure. This procedure includes initially all factors identified as potential predictors of CD and then removes factors not Inter-Hospital Comparison of Caesarean Delivery Rates associated with CD at p,0.05. The reference category for the calculation of the RR was identified using a recursive procedure set up by the P.Re.Val.E. Project [20]. This procedure enables identification of a homogeneous subset of hospitals with the lowest adjusted risk of CD. The procedure was replicated for each of the two study groups. Crude and adjusted RRs were used to rank hospitals and the percentage difference between crude and adjusted RR was calculated. A percentage difference greater than 10% and a change in ranking .3 were considered as relevant [21]. The correlation between crude and adjusted CD rates was assessed using Spearman’s correlation coefficient. C-index and AIC were used to evaluate the goodness of fit of the models. Specifically, the C-index was used to assess how well the model discriminates between women with and without a CD. The area under the curve ranges from 0.50 (no ability to discriminate) to 1 (perfect discrimination). the AIC (Akaike Information Criterion) is a measure that combines fit and complexity of the model. Lower values indicate a better fit of the model taking complexity into account. All statistical analyses were performed using Stata 10 (College Station, Texas 77845, USA). Results During the study period, in the Emilia Romagna Region, in 24 hospitals there was a total of 98,913 deliveries, of which 87,849 had no previous CD and 46,179 were NTCS. The overall CD rate was 30.3% (range 19.2–53.9%, Coefficient of variation-CV: 22.6); the primary CD rate was 22.4%, (range 13.3–40.2%, CV: 25.3) and the NTCS CD rate was 23.4% (range 12.7–42.5%, CV: 27.1). The NTCS CD rate contributed 37.5% of overall CD rate (range 26–46%, CV: 13.6). Table S1 reports crude and adjusted primary CD rates and crude and adjusted RRs for each hospital compared to the reference category. Variables included in the final models and goodness of fit indices are listed as a note to the table. In the Emilia Romagna Region, the primary CD rate in the reference category, including five hospitals, was 15.0%. In the other hospitals, crude CD rates ranged from 16.8% (Hospital M) to 40.2% (Hospital E) while after adjustment, rates ranged from 16.9% (Hospital F) to 31.3% (Hospital E). Compared with the reference category, the adjusted RRs ranged from 1.13 (Hospital F) to 2.09. In women without previous CD, 9/24 hospitals showed a .10% variation between crude and adjusted RRs, and 8 out of those 9 hospitals had their risk reduced compared with the reference category. There was no significant difference on number of deliveries and the presence/absence of training programs between hospital that changed and those that did not change, both for primary CD and NTCS (data not shown). Of the 15 hospitals showing a #10% difference between crude and adjusted RRs, 5 had their risk reduced and 10 increased Table S2 reports the results for the NTCS group. The CD rate in the reference category, including the same five hospitals, was 14.5%, the highest unadjusted rate in Emilia Romagna Region was 42.6% (Hospital E) and the highest adjusted rate was 38.2%. In the NTCS sample, a.10% difference between crude and adjusted RRs was found in 2 hospitals, and in both cases adjusted RRs were lower than crude RR. Six hospitals in the NTCS group had adjusted RRs more than twice as high as those of the reference group. As for hospital ranking using crude and adjusted RR of primary CD, in women without previous CD, 23 hospitals had their rank changed after adjustment: four hospitals differed 1 rank, seven differed 2–3 ranks, twelve differed 4–11 ranks. In the NTCS PLoS ONE | www.plosone.org 2 November 2011 | Volume 6 | Issue 11 | e28060