If we suggest that x causes y, can we be sure that it is x that is responsible for the variation in y ? This relates to :

Whether x rather than something else truly causes the variation in y concerns internal validity.

External validity instead asks whether findings generalise beyond the specific study context.

Reliability concerns the consistency and repeatability of a measure, not causal certainty.

Ecological validity asks whether findings hold in natural, everyday settings.

Internal validity is strengthened by controlling confounding variables and using random assignment.

A tightly controlled experiment usually maximises internal validity at some cost to external validity.