In this situation, the CAL should still be considered as an add-on instructional aid to improve traditional classroom instruction. The  instructor, although no longer the center of attention, continues to maintain adequate control over the learning environment to ensure  learning objectives are being achieved.  [Figure 5-18]  A more advanced application of computer-based training may involve less instructor control. For example, a laboratory-type  environment may be configured with separate study areas for each learner. With this setup, the physical facility is usually referred to  as a learning center or training center.  Learners in these centers are often monitored by a teacher’s aide or other trained personnel who can provide guidance, answer$_{ }$ questions, and act as a conduit to the instructor who is responsible for the training. In this case, the responsible instructor needs to  establish procedures to make sure the required training is accomplished, since he or she certif ie s learner competency at the end of the  course.  Models, mock-ups, and cut-aways are additional instructional aids. A model is a copy of a real object. It can be an enlargement, a  reduction, or the same size as the original. The scale model represents an exact reproduction of the original, while simplified models  do not represent reality in all details. Some models are solid and show only the outline of the object they portray, while others can be  manipulated or operated.  Although a model may not be a realistic copy of an actual piece of equipment, it can be used effectively in explaining operating  principles of various types of equipment. Models are especially adaptable to small group discussions in which learners are  encouraged to ask questions. A model is even more effective if it works like the original, and if it can be taken apart and reassembled.  With the display of an operating model, the learners can observe how each part works in relation to the other parts. When the  instructor points to each part of the model while explaining these relationships, the learners can better understand the mechanical  principles involved. As instructional aids, models are usually more practical than originals because they are lightweight and easy to  manipulate.  A mock-up is a three-dimensional or specialized type of working model made from real or synthetic materials. It is used for study,  training, or testing in place of the real object, which is too costly or too dangerous, or which is impossible to obtain. The mock-up  may emphasize or highlight elements or components for learning and eliminate nonessential elements.  Cut-aways, another type of model, are built in sections and can be taken apart to reveal the internal structure. Whenever possible, the  various parts should be labeled or colored to clarify relationships.  Production and equipment costs are limiting factors to consider in developing and using models, mock-ups, and cut-aways.  Depending on the nature of the representation, cost can vary. For instance, scale replicas are often very expensive. In general, if a  two-dimensional representation will satisfy the instructor’s requirement, it should be used.$_{ }$ 5-27  Models, Mock-ups, and Cut-Aways  Figure 5-18.  In a computer-assisted learning environment, the instructor still ensures that learning objectives are being achieved. 