Professional software packages such as The WinQSB and LINDO provide the following LP information:

• Information about the objective function:– its optimal value– coefficient ranges (ranges of optimality)

• Information about the decision variables:– their optimal values– their reduced costs

• Information about the constraints:– the amount of slack or surplus– the dual prices– right-hand side ranges (ranges of validity of shadow prices)

• In this presentation we will discuss:

– changes in the coefficients of the objective function

– changes in the right-hand side value of a constraint

• Sensitivity analysis (or post-optimality analysis) is used to determine how the optimal solution is affected by changes, within specified ranges, in:

– the objective function coefficients

– the right-hand side (RHS) values

• Sensitivity analysis is important to the manager who must operate in a dynamic environment with imprecise estimates of the coefficients.

•Sensitivity analysis allows him to ask certain what-if questions about the problem.

Consider the following LP problem:Consider the following LP problem:Max 5Max 5xx1 + 71 + 7xx22s.t. s.t. xx1 1 << 6 6 22xx1 + 31 + 3xx2 2 << 19 19 xx1 + 1 + xx2 2 << 8 8

xx1, 1, xx2 2 >> 0 0

• Graphical Solution

88

77

66

55

44

33

22

11

1 2 3 4 5 6 7 8 9 101 2 3 4 5 6 7 8 9 10

xx22

xx11

xx11 + + xx22 << 8 8Max 5x1 + 7x2Max 5x1 + 7x2

xx11 << 6 6

Optimal: Optimal: xx11 = 5, = 5, xx22 = 3, = 3, zz = 46 = 46

Let us consider how a change in the right-hand side for a constraint might Let us consider how a change in the right-hand side for a constraint might affect the feasible region and perhaps cause a change in the optimal affect the feasible region and perhaps cause a change in the optimal solution.solution.The The improvementimprovement in the value of the optimal solution per unit in the value of the optimal solution per unit increaseincrease in in the right-hand side is called the the right-hand side is called the dual pricedual price..The The range of feasibilityrange of feasibility is the range over which the dual price is is the range over which the dual price is applicable.applicable.As the RHS increases, other constraints will become binding and limit the As the RHS increases, other constraints will become binding and limit the change in the value of the objective function.change in the value of the objective function.

The The range of RHSrange of RHS for a change in the right hand side value is the range of for a change in the right hand side value is the range of values for this coefficient in which the original dual price remains constant.values for this coefficient in which the original dual price remains constant.Graphically, the range of feasibility is determined by finding the values of a Graphically, the range of feasibility is determined by finding the values of a right hand side coefficient such that the same two lines that determined the right hand side coefficient such that the same two lines that determined the original optimal solution continue to determine the optimal solution for the original optimal solution continue to determine the optimal solution for the problem.problem.

Another Example: Max 10Another Example: Max 10xx1 + 15x2 (Total Weekly Profit) 1 + 15x2 (Total Weekly Profit)

s.t. 2s.t. 2xx1 + 41 + 4xx2 2 << 100 (Aluminum Available) 100 (Aluminum Available) 33xx1 + 21 + 2xx2 2 << 80 (Steel Available) 80 (Steel Available)

xx1, 1, xx2 2 >> 0 0

Optimal SolutionOptimal Solution

According to the WinQSB output:According to the WinQSB output: xx1 (Deluxe frames) 1 (Deluxe frames) = 15= 15 xx2 (Professional frames) 2 (Professional frames) = 17.5= 17.5Objective function value Objective function value = $412.50= $412.50

QuestionQuestionSuppose the profit on deluxe frames is increased to $20. Suppose the profit on deluxe frames is increased to $20. Is the above solution still optimal? What is the value of the Is the above solution still optimal? What is the value of the objective function when this unit profit is increased to $20?objective function when this unit profit is increased to $20?

AnswerAnswerThe output states that the solution remains optimal as long as the objective The output states that the solution remains optimal as long as the objective function coefficient of function coefficient of xx1 is between 7.5 and 22.5. Since 20 is within this 1 is between 7.5 and 22.5. Since 20 is within this range, the optimal solution will not change. The optimal profit will change: range, the optimal solution will not change. The optimal profit will change: 2020xx1 + 151 + 15xx2 = 20(15) + 15(17.5) = $562.50.2 = 20(15) + 15(17.5) = $562.50.

QuestionQuestionIf aluminum were a relevant cost, what is the maximum If aluminum were a relevant cost, what is the maximum

amount the company should pay for 50 extra pounds of aluminum? amount the company should pay for 50 extra pounds of aluminum?

AnswerAnswerSince the cost for aluminum is a sunk cost, the shadow price provides the Since the cost for aluminum is a sunk cost, the shadow price provides the value of extra aluminum. The shadow price for aluminum is the same as value of extra aluminum. The shadow price for aluminum is the same as its dual price (for a maximization problem). The shadow price for its dual price (for a maximization problem). The shadow price for aluminum is $3.125 per pound and the maximum allowable increase is 60 aluminum is $3.125 per pound and the maximum allowable increase is 60 pounds. Because 50 is in this range, the $3.125 is valid. Thus, the value pounds. Because 50 is in this range, the $3.125 is valid. Thus, the value of 50 additional pounds is = 50($3.125) = $156.25.of 50 additional pounds is = 50($3.125) = $156.25.

Let us consider how a change in the right-hand side for a constraint might Let us consider how a change in the right-hand side for a constraint might affect the feasible region and perhaps cause a change in the optimal affect the feasible region and perhaps cause a change in the optimal solution.solution.The The improvementimprovement in the value of the optimal solution per unit in the value of the optimal solution per unit increaseincrease in in the right-hand side is called the the right-hand side is called the dual pricedual price..The The range of RHSrange of RHS is the range over which the dual price is applicable. is the range over which the dual price is applicable.As the RHS increases, other constraints will become binding and limit the As the RHS increases, other constraints will become binding and limit the change in the value of the objective function.change in the value of the objective function.