diff --git a/src/model/validity.py b/src/model/validity.py
index 401ed29..2bdace3 100644
--- a/src/model/validity.py
+++ b/src/model/validity.py
@@ -26,6 +26,12 @@ METHOD_RANKINGS = {
     5.0: ["RCT", "randomi(?:s|z)ed.control.trial"],
 }
 
+VALIDITY_STRENGTH_BINS = {
+    0.0: r"\-",
+    5.0: r"\+",
+    10.0: r"\++",
+}
+
 
 def calculate(
     df: DataFrame,