1. Vertical distances between farm level supply and retail demand (plotted in the same diagram) are used by Fisher to map a demand for marketing services.
2. Even though watermelons are sold at the retail level often as cutup products, it is fairly easy to think of the total quantity (measured either in pounds or total number) of watermelons sold at the retail level to be essentially the same quantity produced and sold at the farm level. Adjustments for shrink, spoilage and other quantity losses as well as adjustments for the value of by-products resulting from the marketing process need to be acknowledged and must be incorporated as part of the difference between retail and farm prices for any particular total quantity of farm or retail product sold. Another subtlety involved in linking retail and farm level demands is determining a correspondence between farm and retail products. Considerable aggregation of derived demands associated with different retail products may be required to consider the aggregate demand for any particular agricultural product used in the production of a variety of retail products.
3. The quantity dimension used for expressing prices per unit on the vertical axis of such diagrams frequently is identical to that selected for the horizontal axis, but does not have to be. For example, the horizontal axis could be expressed in tons, but prices might be dollars per lb. or expressed in terms of some other quantity unit. The critical issue for such diagrams is that prices at various levels of the marketing system be comparable in order for vertical distances for particular quantities to be economically meaningful. Gardner's article examines price spreads and relative price ratios that are economically meaningful if quantity units are identical or remain in a fixed proportion.
4. The Farm‐Retail Price Spread in a Competitive Food Industry