Chung 2010.

Study characteristics
Methods	Controlled before‐after study
Participants	Primary care physicians
Interventions	California, USA Intervention:physician‐specific P4P plus existing payment (salary based on relative value units of service). The bonus amount was based on individual physicians' performance on 15 ambulatory quality measures, with a composite score calculated using an algorithm developed by the incentive programme leadership. The physicians set targets for each measure. Physicians received varying points for achieving minimal, average, and stretch goals based on the percentage of their patients achieving the target. The bonus was based on the percentage of potentially achievable points actually earned. The maximum achievable bonus was USD 5000/year, or about 2% of the primary care physicians annual salary. Control: existing payment (salary based on relative value units of service). The other 2 physician groups continued using the previous P4P measures for group‐level or department‐level performance, but no group‐level or department‐level bonuses were distributed to individual physicians. Years 2005 to 2007, intervention happened in 2007
Outcomes	Quality of health services provided Quality score for asthma controller prescribing: no difference Quality score for cervical cancer screening: no difference Quality score for chlamydia screening: no difference
Notes	There are only figures for the quality score in the control group but no specific data. Controlled before‐after study with high risk of bias
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	High risk	Control before‐after study rated as 'high risk' for this domain.
Allocation concealment (selection bias)	High risk	This is a controlled before‐after study (years 1998 to 2001, intervention happened in May 1999).
Baseline outcome measurements similar	Unclear risk	No comparison on the baseline outcome measurements was reported.
Baseline characteristics similar	Low risk	All 3 physician groups, located at clinics in adjacent counties, had a roughly similar mix of primary care and specialty physicians and served patients of similar demographic composition.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Amongst 179 physicians, 167 were included in the study, whilst 12 had insufficient qualifying patients for various reasons. Most physicians (152 in 2005 and 169 in 2006) also had data for the equivalent measures in the previous years; 148 had data for all 3 years
Knowledge of the allocated interventions adequately prevented	Low risk	The outcomes are objective.
Study adequately protected against contamination	Low risk	Allocation was by county, and it is unlikely that the control group received the intervention.
Selective reporting (reporting bias)	Unclear risk	Not specified in the paper
Other bias	High risk	Did not have a contemporaneous comparison group at the same study site receiving only performance reporting or group‐level incentives