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Table 1

Two main sources of biases threatening internal validity of SAT/ECA studies.

Patient-level Inconsistent measurement of endpoints (e.g., uneven follow-up time of long-term progression events and overall survival)/prognostic risk factors between SATs and ECAs.
Non-random missingness of data.
Differences in observed baseline characteristics that make it challenging to define SoC for ECAs as a comparator.
Differences in unobserved baseline characteristics.
Use of historical (vs. concurrent) ECAs* in which practice may deviate from that in SATs.
System-level Underlining heterogeneity among the systems**.
 −Population-level disease burdens.
 −Practice style, experience, reimbursement policy, clinical guideline.

Historical ECAs refer to patients in SATs and ECAs are drawn from difference time periods.


For example, one system may consist of a set of hospitals from which SAT patents are drawn whereas the other consists of an entirely different set of hospitals from which ECA patients are drawn.

SAT: single arm trial; ECA: external control arm; SoC: standard-of-care.

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