Part one. Summarise
People rely on consistency. It enables people to be able to efficiently transfer knowledge, learn new things quickly, and focus attention on similar tasks. There are four different types of consistency. Aesthetic consistency is a logo on a car or product. People look at the logo and know to expect good or bad things about it. Functional consistency is about actions. People know that when a traffic light goes amber, to expect it to go red. Or when a light goes red, they should expect it to turn green again, soon. Internal consistency is similar to other elements in the system, like signs in a park. External consistency is consistency relevant to the environment, like security/fire alarms. You must consider Aesthetic and functional consistency in all aspects of design.
According to Hassan, Sayed and Tabernero (2001), design consistency could be described as the way highway systems are designed to avoid traffic accidents. This could be functional consistency, since it is easier to use.
Consistency is also one of the most important aspects of design for interactive systems. It is quite often listed as encouraging to use. Almost every usability-expert will say consistency is important. Kellogg (1989), points out, “consistency has no meaning on its own; it is inheriently a relational concept. Therefore to merely say that an interface is consistent or that consistency is a goal of user interface design is also meaningless.” (Obendorf, 2009).
Hassan, Y., Sayed, T., & Tabernero, V. (2001). Establishing practical approach for design consistency evaluation. Journal Of Transportation Engineering, 127(4), 295–302.
Lidwell, W., Holden, K., & Butler, J. (2003). Aesthetic-Usability Effect. In Universal Principles of
Design (pp. 46). Massachusetts: Rockport.
Obendorf, H. (2009). Minimalism. Dordrecht [The Netherlands]: Springer.