It is very easy to assume that one economic variable causes the change in another. For example, that the change in the price of a product results in a change in the quantity demanded. The reality, however, may be that the demand for the product has changed -- and as a result there has been a change in price.

It is important to remember that economic variables may be closely correlated -- but this does not in any way support the believe that a change in one variable is the cause of the change in another.