Table:

Summary of type 1 and type 2 selection bias

Selection bias	Type 1	Type 2
Definition	Restricting to one or more level(s) of a collider (or a descendant of a collider)	Restricting to one or more level(s) of an effect measure modifier
Other names	Collider stratification bias; collider restriction bias	generalizability bias
Can occur on sharp null?	Yes	No
Bias in the referent population?	Yes	Yes
Bias in the selected sample?	Yes	No
Can affect internal validity?	Yes	Yes
Can affect external validity?	No	Yes
Effect measure scale dependent?	No	Yes