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Health Services Research (HSR) Methods
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Selection Bias
Synonyms: Omitted Variable Bias; Self-Selection Bias; Unmeasured Confounding; Unobserved Confounding; Confounding by (Contra-)indication; Residual Confounding; Hidden Selection Bias

The estimated treatment effect estimate does not represent just the unique effect of treatment on an outcome, because a subset of unobserved confounders non-random assignment of the subjects to the treatment and control groups resulted in the treatment effect estimate.