And what kind(s) of validity is "observer bias" threat to, anyway?

And what kind(s) of validity is "observer bias" threat to, anyway?

Answer: Observer bias is mostly a threat to external and construct validity, but can also be a threat to internal validity in that any observed correlation might be coming from the beliefs of the observers, instead of from a causal relationship between the measured variables. In short, while it's considered a "secondary threat" for both types of observational method, it can be devastating when it occurs.