Marshall 2002

Methods	ITS with monthly data points
Participants	Senior beneficiaries in provincial drug benefit plan, British Columbia, Canada
Interventions	PA policies for PPIs in the context of reference based pricing of H2RAs
Outcomes	Drug use, drug costs
Notes	Analysis from January 1993‐May 1999
Risk of bias
Bias	Authors' judgement	Support for judgement
Incomplete outcome data (attrition bias) All outcomes	Low risk	Any undocumented difference in the proportion of missing data in the administrative datasets pre‐ and post‐intervention is unlikely to overturn study results notwithstanding that differences in completeness of data may have varied over time as a new administrative drug database was implemented coincident with the policy under evaluation
Selective reporting (reporting bias)	Low risk	All relevant outcomes listed in the methods section are reported in the results section
Other bias	Unclear risk	"The reasons for the observed growth in PPI use are unclear and are beyond the scope of this analysis." Overall trends in the drug market could have been examined with data from another province as a control.
Knowledge of allocated interventions adequately prevented during the study	Low risk	Outcomes are objective measures of healthcare utilization
Intervention independent of other changes	Low risk	Known and unknown changes in the environment are unlikely to be responsible for sudden change and magnitude of effect size All prescriptions for PPIs issued by gastroenterologists were automatically exempted from the special authority policy.
The shape of intervention pre‐specified?	Low risk	The point of analysis is the point of intervention; ie, the date the policy intervention was implemented was used to delineate pre and post policy time periods with adequate data points to capture the shape of the pattern of intervention effect over time
Intervention unlikely to affect data collection?	Low risk	Aggregate monthly claims data for H2RAs, PPIs, sucralfate, prokinetic agents and misoprostol were provided by British Columbia Pharmacare for the period January 1993 to May 1999, inclusive, to cover periods before and after introduction of the reference‐based pricing and special authority policies