Substitution is used in nutritional studies as a better metric than simply eating a food item.
Eating olive oil, for example, means a deliberate choice to not use animal fat, butter or mayonnaise for the same purpose.
The nutritional change is not simply the addition of olive oil to the diet but the removal of other items which may offer specific nutrients not found in olive oil.
Replacing margarine, butter mayonnaise and dairy fat with the equivalent amount of olive oil produced an 8-34 percent lower risk of death depending on the cause.
No significant associations were found in the study for replacing other vegetable oils with olive oil.