["In England, patients can choose to have their NHS elective care delivered by private (or 'independent sector') providers or by the NHS. Providers are paid a fixed tariff for each type of procedure. Our objectives were to compare NHS providers with private treatment centres in terms of (a) the quality of data coding and (b) patient complexity.", 'We compared elective patients aged 18 years and over treated in the NHS and private sectors using the Hospital Episode Statistics (HES) data for 2007-2008. The absence of diagnostic information was used as a measure of data quality. We analysed differences in complexity for each of the 30 Healthcare Resource Groups (HRGs) that together account for 78% of coded private treatment centre activity. Statistical significance was assessed at the 1% level.', 'Hospitals and treatment centres.', 'Patient complexity was assessed by four characteristics: age; number of diagnoses; number of procedures; and income deprivation of residential area.', 'NHS providers treated almost 7 million adult elective patients in 2007-2008. Fewer than 100,000 patients were treated by private providers (1.3% of elective activity). Less than 1% of NHS patients lacked diagnostic information compared to 36% of patients treated by private providers. For the top 30 HRGs, NHS patients had significantly (p<0.01) higher levels of co-morbidity, underwent more procedures and were more likely to come from deprived areas compared with patients treated by private providers. Although patients treated in private settings tended to be younger, the difference was not statistically significant.'] Are private sector treatment centres treating less complex patients than the NHS?