The importance of consistency, and what consistency really means with regard to software design, has to be one of the most misunderstood and misapplied principles. I am constantly tortured with “well, Microsoft does it like this – why don’t we just copy that? It would be consistent with what users expect.”, and “let’s make this control look like this other one to be consistent”.

With this modus operandi, how can you ever expect to innovate?

Consistency is not about copying existing designs to shortcut necessary creative work (Note: I’m not talking about looking to other designs for inspiration). True, this can be used effectively, and may even be a good idea in some cases – there are many great designs out there worthy of mimicry! But it should not be a standard practice. Should every car look like a Honda? Should every tool in your garage have the same handle?

Should form follow function, or form follow form?

Maybe we should create a standards committee that specifies how all buttons should look and behave; then when someone develops a new software application, they can go to a website and download all the controls they need to build it. Over time all software products will look and behave exactly the same and our users will be much happier, right? I don’t think so.

And even more importantly, consistency does not mean sacrificing usability for the sake of code reuse! Again, I am a practical designer; I understand the various business and engineering considerations (time to market, cost, etc.). I’m not advocating reinventing the wheel. But I certainly don’t think that a bicycle, motorcycle, van, or high-performance sports car should all share the same wheel design either! Code reuse should be leveraged whenever possible but it has to be weighed against other considerations such as basic usability, existing metaphors and mental models, etc.

So, how can one apply consistency correctly? By thinking in terms of “conceptual integrity,” a slightly more abstract concept coined by Frederick Brooks in The Mythical Man-Month. CI means that the software system as a whole should reflect one vision and should fit together seamlessly and flow naturally. This will require consistency throughout the system, but it will not be the driving force. The usage of the system is the first consideration and consistency is applied to support this.

Form follows function and then goes through the consistency/best-practices and usability testing ovens to create the final product.

Related posts:

1. The same, or different, but not similar!
2. What do we mean by software design?
3. Why usability is a path to failure (or “Usability is not design”)