Study On Constraints In Application Design Computer Science Essay

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Norman describes that the physical constraints limit possible operations as they are operated based on properties of physical world like size, shape etc. Semantic constraints are based on the meaning of the situation to limit the number of possible actions and are dependent on the user's knowledge of the situation and of the world. Cultural constraints are based on a set of accepted cultural conventions for social situations. This knowledge is acquired by schemas and knowledge structures having general rules and information to interpret situations. Logical constraints use some logic to limit the actions in terms of Natural Mappings (left-right, up-down), Sequence (actions performed in obvious order) and Completeness (using all parts, filling all fields). A good example of building Lego motorcycle is illustrated to explain these concepts.

The author clearly explains the concepts of constraints, affordances and mapping by applying them to the design of real world objects like doors and switches. The regular designs of them confuse the users because of the lack of enough clues and visible signals that prompt for the right operation. He says that if a simple design needs instructions or pictures, it is likely a bad design. The design should effectively solve the mapping and grouping problems that occur.

Two more important principles for knowing what to do in situations are Visibility and Feedback. Relevant parts like system states and controls should be made visible using displays and/or sounds for better designs like doors must have handles to operate. If the capabilities are visible on the design, it does not need memory on how to use and if the functionalities are hidden, problems occur. The design and usability of an object is greatly improved by making the invisible parts of it clearly visible.

The instant information of any system related to the performance or the validation of the projected results can be achieved through a feedback mechanism. It is essential factor that verifies if the task is successfully completed like good visual display for visual feedback. Immediate response and obvious effect for each action signals the user on what is taking place. For instance it could be a sound that's made or could be a change in a physical state. For those things that can't be made visible, sounds serve as the best feedback. They even grab the user's attention to convey the information, which is indeed useful but deficit at times. Often silence may raise problems when sound feedback is expected. However, for a good conceptual model for better design, a designer should effectively incorporate all these concepts in the design.

Part 2 - Description of various concepts with examples


Constraints limits to the apparent operation of any piece of equipment. It suggests the accurate actions so that the things should fall into right place. On a daily basis constraints can be classified as physical, semantic, cultural, and logical.

Physical constraints are the physical limitations that restrict possible events. There is no particular guidance required and the value of physical constraints depends on the properties of physical world.

Semantic constraints are the limitations based on the meaning of the situation which controls the set of feasible actions. Fundamentally these constraints depend on our knowledge of the circumstances and of the physical world.

Cultural constraints come into picture when we deal with new devices and these constraints are the basis for many problems which depends on accepted cultural conventions.

Logical constraints are nothing but using logic to constrain actions like natural mappings, to follow a sequence by placing or doing things in a correct order, and also using all the given parts in order to achieve the completeness.


Positive Example:

Inserting a sim card into a mobile device is a good example for constraints, as the shape of the sim card exactly fits into the designed shape of the sim tray in the mobile phone.

In this example, constraints are clearly used to limit the physical space available to fit specific shape of sim card. It suggests the accurate action needed so that the sim card fits in the right place. This uses the physical constraints and semantic constraints where it depends on the knowledge of the world about cell phones.

Negative Example:

Iphone is a good example where constraints are not effectively used.

While talking to a person on iPhone, the sensitive touch feature of it either ends up the call abruptly or the mute/ speaker/ hold buttons gets clicked automatically without the user's notice, as the keypad cannot be locked while the user is on call. Even if the user presses the keypad lock on the top, the call ends up. And if a phone is handed to the other person while on call, and if the user unknowingly touches the screen, the call might get ended or any action on the screen could take place. It would be difficult for a completely naïve user to talk on an iPhone. On the other hand, the user can lock the keypad and talk on the phone without any issues, if he uses the head phones.

In order to constrain the touch feature while on call, the iPhone should have some feature of locking the keypad which doesn't end up the call.


Correct parts must be visible and they must convey the correct message to indicate what parts operate and how, so that the user can interact easily with the device. It indicates the mapping between intended actions and actual operations.

It is very obvious that when the facilities are visible, anyone can know how to implement things without the need of any sort of memory. In other words if the right parts which also conveys correct information to operate a machine are visible, a user can easily operate it without any extra memory or additional guidance. Here comes the concept of natural signals which is the common understanding of objects and their observable use and natural design that takes advantage of these signals.


Positive Example:

All the correct parts for the operation of car before start are clearly visible on the key and they convey the correct information using pictures to indicate the respective operation. Both the above car remotes are good examples for visibility, as they easily make the user understand which button is for which control/operation. The lock symbol or 'LOCK' label easily lets him understand that it is used to lock the doors of the car and similarly the remaining buttons also convey its intended operation to the user. Also the user needn't use his memory to memorize the functions on it. It maps the planned action and the actual operation of unlock/lock/opening trunk.

Negative Example:

In this example, the correct parts of the above refrigerator are not placed or visible at the correct location of the operation. It has handles on both the sides and lacks visibility as it does not convey the user about which side to use or to open the door. It seems confusing to the user. Whenever the user wants to open it, he has to recollect from his memory when he last used it on how to operate it. This design hinders its observable use and natural design. It would be more visible and good design if the handles are placed on the front of the doors on one side.


Feedback mechanism is the immediate and obvious effect of the information to the user about the result of a process or an activity. It also gives a clear picture to the user like what action has been done and what result was accomplished. It may be in terms of sound or visual feedback. It is also a prominent concept in control system engineering and information theory. Feedback has many interesting properties that can be developed in designing various systems and it always behaves in a counterintuitive manner.


Positive Example:

The rice cooker gives good feedback to a user by switching on the 'warm' light present on it, when the rice/vegetables gets cooked and by switching on the 'cook' light when the rice is getting cooked. It gives an immediate response using a either a light signal or sound signal as feedback. It clarifies that the operation has been finished. The user need not manually lift up the lid to check if the rice is cooked. The LEDs present on it automatically tells the user about its function.

Another good example of feedback is the smoke detector. When there is a fire accident or smoke in a room or house or a building, the alarm of the detector rings by itself and also a red light switches on automatically that intimates the fire station. This clearly gives immediate and obvious effect through sound signal intimating the user to respond quickly.

Negative Example:

Even now, this old iron box is being used in many rural places, which is run by charcoal. It does not have any light or sound indicator when the job is finished. Or in other words, there is no feedback signal from this device that tells the user about the completed job. One should understand its mechanism and be able to judge if the product is heated enough. There is no obvious result of this process that indicates the iron box is over heated. A sound or light signal is expected in this situation and this silence is causing dangerous fire problems.