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Table 1 IQWIG guidance [5] on interpretation of validation studies of surrogate outcomes

From: Surrogate endpoints in oncology: when are they acceptable for regulatory and clinical decisions, and are they currently overused?

Strength of association R-value
Validity proven Lower limit of 95% CI ≥ 0.85
Unclear validity R < 0.85 to > 0.7
Proven lack of validity Upper limit of 95% CI ≤ 0.7
  1. The R-value is the correlation coefficient derived through methods such as Pearson product moment, Kendall’s Tau or Spearman’s rank correlation coefficient