Marginal analysis

The Marginal Analysis page gives you a detailed breakdown of the figures found in the Marginal column of the Purchases page, for those purchases that have hit an active constraint.

Note: For the Marginal Analysis page to be populated, you must select Marginal Analysis in the Result Retrieval Settings page of the Run Settings dialog.

Purchases for which marginal analysis is carried out are listed on the top header row next to the Effect header. For each of these purchases there are two columns:

• Delta shows the change in sold or purchased units of a given material. The amount of a unit depends on the unit of measure in the UoM column (for example, a barrel of crude oil, or a gallon of gasoline).

• Marginal shows the effect of the change in purchased or sold quantities on the objective function. For example, one extra barrel of Agbami will decrease the objective function by the cost per barrel of this crude.

The External row show any changes from items that are not listed in the table, such as utilities. If you were editing a plant from a Network case, any changes in purchases or sales originating outside the plant would contribute to the External figure.

The Total row shows the combined effect on the objective function of all the changes in purchases and sales listed in each Marginal column. The numbers on the Total row are the same as those in the Marginal column of the Purchases page.

Consider the following example:

The previous figure shows the marginal analysis for an Agbami purchase that has hit a maximum constraint. To calculate the marginal value, the constraint is violated by purchasing an extra unit (in this case an extra barrel) of Agbami. This is shown on the first row, with the objective function decreasing by $110.55, which is the price per barrel of Agbami as shown in the Price column.

This extra purchase causes changes in a number of other purchases. For example, the purchase of Lower Zakum decreases by 1.239 barrels, which translates into a saving of $136.23.

As for sales, the main effects are a decrease in the sale of premium gasoline, with a loss of $11.837, partially compensated by increased sales of Ultra Low Sulfur Diesel (ULSD) for a gain of $2.904.

The overall net effect is that an extra barrel of Agbami causes the objective function to increase by $1.272.