Table 2. . Synthetic control quality checklist – external control data source questions.

Item number	Key question	Criteria for judgement
1	Was the original data collection process similar to that of the clinical trial?	State whether patients are from large, well-conducted randomized controlled trials or high-quality prospective cohort studies, and whether patient characteristics are similar to the target population
2	Was the external control population sufficiently similar to the clinical trial population?	State how the external population is similar with regard to key characteristics, such as (but not limited to): age, geographic distribution, performance status, treatment history, and sex
3	Did the outcome definitions of the external control match those of the clinical trial?	State whether the outcomes are measured similarly or not
4	Was the synthetic control dataset sufficiently reliable and comprehensive?	State whether there are sufficient sample sizes and covariates that can create comparable control groups
5	Were there any other major limitations to the dataset?	State any other aspects of the dataset that would limit the reliability and validity of comparisons

Adapted with permission from [10].