When designing a form, should I favor consistency over optimal usability for each input?

I was reading this question and this thought occurred to me:

When I am designing medium or longer length forms in a web interface I often favor consistency in choosing the input mediums for the information I am capturing instead of varying the input mediums in an attempt to optimize the usability of each individual input. I do this because I have always believed that a consistent interface increases usability.

An example of this might be a form where a user must make several categorical selections. I might choose to use drop down elements for all the choices even when a button group might work better for an input that only has a few possible choices, just so the input interface feels more consistent:

^{download bmml source – Wireframes created with Balsamiq Mockups}

It occurred to me that I don't really have anything more than anecdotal evidence to support this position. Is there any research or case studies that support this position (or the reverse position)?

Answer

That's the (nearly) clear-cut answer to your question. Phrased alternatively:

Consistency follows usability.

Allow me to explain...

There isn't an agreement on what usability is exactly. Here are a few definitions:

ISO 9241

Usability is measured by:

• Effectiveness - the extent to which the intended goals of use of the overall system are achieved.


• Efficiency - the resources that have to be expended to achieve the intended goals.


• Satisfaction - the extent to which the user finds the overall system acceptable.

Nielsen

Jakob Nielsen defines usability like so:

• Learnability - how easy is it for users to accomplish basic tasks the first time they encounter the design?


• Efficiency - once users have learned the design, how quickly can they perform tasks?



• Memorability - when users return to the design after a period of not using it, how easily can they reestablish proficiency?


• Errors - how many errors do users make, how severe are these errors, and how easily can they recover from the errors?


• Satisfaction - how pleasant is it to use the design?

Universal Principles of Design

The book defines usability as:

How easy and forgiving a design is to use.

Izhaki's definition

All these definitions can be criticised. My favourite is the latter where 'easy' is interpreted as the ISO definition of 'the resources that have expended to achieve the intended goals'.

And so resources can be defined as:

• Time - he time it takes to achieve the goal.


• Cognitive effort
  
  
  
  • Interpretation - to interpret the system output.
  
  
  • Decision - to make a decision.
  
  
  • Learning - the time it takes to learn the system, which breaks into:
    
    
    
    
    • Immediate usage.
    
    
    • Repeated usage (ie, retention).
    
    
    
    
  
  
  
  


• Physical effort - such as that involving moving the mouse, or typing on the keyboard.

Regardless of what definition you favour, the whole point of consistency is to expose users to familiar patterns, by that reducing cognitive effort.

In other words - consistency is a tool to increase usability. The only question left is whether an inconsistent pattern would increase usability more than a consistent one. In your case this is clear cut - there is nothing users shouldn't be able to work out, or be confused about.

Although I don't have the source at the moment, there has been a research showing that designers are much more sensitive to consistency than users - if users get it, they care little whether it's consistent or not.

If you come to think about it, any novel design must be inconsistent; but it may still deliver a great value (including higher usability). If we had to always stay consistent, we would not be able to improve.

If you look at this very page, the share/edit/close/flag link are colourless. This is against the standard, where users expect colours to signify interaction elements, and would be considered 'inconsistent' with an overwhelming amount of sites.

However, the designers of this site prioritised eye guiding over consistency. An assumption was made that a user wishing to perform a task will find the related element regardless of its colour. This is pretty smart and it credits users as intelligent beings, rather than consistency-machines. It also utilise the one-step-learning techniques (all a user has to do is try it once, and they'll remember it forever).

An interesting fact about this colourless links is that when I've asked people "Do think this is a good design?" the next-to-unanimous reply was: "No, it's confusing and it makes the text look like a part of the post". But if I asked "Say the grammar in this post was really bad and you'd like to correct it, how would you do that?" - everyone moused to the edit link with seemingly little struggle.