The PLC model is of some degree of effectiveness to marketing professionals, in that it is dependant on factual assumptions. Nevertheless, it is difficult for marketing management to determine accurately where a product is on its PLC graph. A rise in sales per se is definitely not evidence of progress. A fall season in sales by itself will not typify drop. Furthermore, some products do not (or even to date, at the least, have not) experienced a decrease. Coca Cola and Pepsi are types of two products which have existed for many decades, but are still popular products all over the world. Both methods of cola have been in maturity for a few years. Another factor is the fact that differing products would possess different PLC "shapes". A fad product would hold a steep sloped development stage, a short maturity stage, and a steep sloped decrease stage.