Consistency refers to the principle in which systems are more usable and learnable when similar parts are expressed in similar ways. There are four types of consistency. Aesthetic consistency refers to the consistency of style and appearance which enhances recognition, communicates membership, sets emotional expectations. Functional consistency refers to the consistency of meaning and action. It takes existing signs and symbols and applies them to new designs and hence improves usability and learnability, enabling people to leverage existing knowledge about how the design functions.
Internal consistency is consistency with other elements in the system which builds trust wit the user while external consistency is consistency with other elements int the environment and is seen as an extension of the benefits of internal consistency.
Visual design can be defined in terms of both difference and continuity, whereby it means difference because it expresses a company or person’s individuality, but it is consistent as it testifies to the ongoing industrial, economic and social values of the company,” (Floch. 2000, p. 33). This use of continuity is key to the relatability and usability of a system. Many researchers have endeavored to find exactly how related consistency and usability are, focusing specifically on web design. In 1997, to demonstrate the harm inconsistency can induce, an experiment on sixty subjects was conducted. It was found that “inconsistent interface terminology slowed user performance by 10-25%”. (Mahajan & Shneiderman, 1997, p.722).
In a separate study, it was found that visual aspects like consistent colours, type of font and the presence of easily accessible navigation bars correlated with the expert quality rating of a site (Van der Geest & Loorbach, 2005, p.27). Hence, there is a clear relationship between the consistency of visual elements in a website and the positive user experience. However, this need for consistency has also been found to limit designers, as it means “each piece of information cannot be maximally distinct.” (Ware, 2008. Pg. 20), and hence web designers are forced to prioritise visual elements in order to give the correct amount of weight to each element while keeping consistency.
Floch, J. O. P. V. F. (2000). Visual Identities. London: Bloomsbury Publishing PLC. Retrieved from https://ebookcentral.proquest.com/lib/ECU/detail.action?docID=436448
Lidwell, W., Holden, K., & Butler, J. (2003). Aesthetic Usability Effect. In Universal Principles of Design (pp. 46). Massachusetts: Rockport.
Mahajan, R., & Shneiderman, B. (1997). Visual and textual consistency checking tools for graphical user interfaces. IEEE Transactions on Software Engineering, 23(11), 722-735.
Van der Geest, T., & Loorbach, N. (2005). Testing the visual consistency of web sites. Technical communication, 52(1), 27-36.
Ware, C. (2008). Visual thinking for design (Morgan kaufmann series in interactive technologies). Burlington, MA: Morgan Kaufmann.