Engineering Optimization

28Class Variables

28.1 Introduction

Class variables are alternately called category, text, nominal, or string variables. They do not have numerical values. They are items or choices. For instance, “Which is better: a car, a motorcycle, or an airplane?” Here the DV is a method of transportation, and the assessment of best could be related to fuel and insurance cost, joy of travel, number of friends you can bring, operator training cost, speed of transportation, parking upon arrival, etc. The decision variable, the list of items, does not have a common, quantifiable metric (either continuum‐valued or discretized) that defines the OF. In size, the airplane is largest and motorcycle smallest, but this aspect is not what defines the OF. The motorcycle might accelerate the fastest, and the car carries the most luggage per passenger, but, again, these are consequences of the DV.

Here are some more examples of choices that are an either–or situation. Which is better: a chair or a stool? Which is better: a ladder, stairs, rope‐and‐pulley, or an elevator? A turbine, piston, or centrifugal pump? Distillation, crystallization, or pressure‐swing absorption? Jim, Ted, or Brad? An electrical engineer or a computer scientist?

Additionally, the list of items may constitute all of the things that need to be completed, and the question is “What is the best sequence of implementing the operations?” For example, in preparing a batch reaction, components A, B, and C need to be added, the material ...