The two divisions do not have a productive or causal effect on each other.

Placental insufficiency as a result of smoking has been shown to have a causal effect in the development of pre-eclampsia.

If the sample were larger and the variance were less, the average causal effect would be closer to the true average causal effect.

The authors state a researcher can make these bold, causation statements while allowing for variance by using what they call variance of causal effect.

Knowing the average causal effect and having observed the response to one treatment for each unit, one can determine the response to the other treatment.