Observational studies and experiments

The method of data collection has a major influence on whether a relationship can be interpreted as causal.

Observational study
There is usually the potential for a lurking variable to underlie any observed relationship, so it is difficult to interpret relationships.
Experiment
Provided the experiment has been designed well, there is little chance of lurking variables driving the observed relationships, so any relationship will be causal.

In a badly designed experiment however, lurking variables can still cause difficulties in interpreting relationships.