|Three forms of case-mix adjustment used in analysis|
|Unadjusted||The simplest models, although accounting for the effect of clustering, used no additional case-mix adjustment.|
|Adjusted||These models additionally controlled for residual imbalances in a set of baseline characteristics. This set included age, sex, ethnicity, site, number of chronic health conditions, principal long-term condition (diabetes, chronic obstructive pulmonary disease or heart failure), an area-based socioeconomic deprivation score (national quartiles of the Index of Multiple Deprivation 2007), and a metric corresponding to the endpoint (e.g., general practitioners contacts) calculated over several periods within the two years prior to recruitment.|
|The number of chronic health conditions was a count of diagnoses recorded on inpatient data over the three years prior to starting the trial. Principal long-term conditions were assigned using a pragmatic approach according to published criteria .|
|Combined model||More complex case-mix adjustment was conducted using the Combined Predictive Model  a standard instrument designed to estimate the probability that an individual will experience an emergency hospital admission in a future twelve month period. The Combined Model score uses 72 variables covering age, sex, recorded health conditions, prior hospital use and prescribing, but not primary care contacts. These variables are sourced from general practice and hospital administrative data. Where a general practice did not grant approval to extract data for the evaluation, or where scores could not be calculated, scores were imputed for its patients based on the available information, which included age, sex and the hospital variables. Single imputation was used based on linear regression on the logit scale. When used in the case-mix adjustment, the Combined Predictive Model score was calculated for each participant at the end of the month prior to the start date.|
Bardsley et al.
Bardsley et al. BMC Health Services Research 2013 13:395 doi:10.1186/1472-6963-13-395