I just re-read Jared Spool's 2005 article Consistency in Design is the Wrong Approach which was referenced in a recent IxDA discussion on consistency.
I deal with the consistency issue fairly often and it is usually pushed for as a quick superficial solution. That is, who the users are and what they are aiming to achieve in using the interface is ignored b/c that is too hard and time-consuming to think about. When in doubt, make it the same. It's my responsibility as the interaction designer to go deeper and determine a design solution that matches the user's mental model.
Some bits from Jared's article:
When you think about consistency, you’re thinking about the product. When you’re thinking about current knowledge, you’re thinking about the user. They are two sides of the same coin. We’ve just noticed that the designers who spend more time thinking about the users are the ones that end up with more usable designs.
Funny thing about thinking about current knowledge: when you’re done, your interface will feel consistent. Why? Because it will match the users’ expectations and, where they expect it to behave like something they’ve encountered before, it does.
Some bits from the IxDA discussion:
From Christine Wodtke:
It's like simplicity; be as consistent as possible and no more. If you had to choose between being appropriate and consistent which would you
From Chauncey Wilson:
The issue here is that there are many types or levels of consistency
that you need to consider. The basic types of consistency are:
Internal consistency - consistency in layout, controls, colors,
commands, features, sizes of objects and areas, and branding
External consistency - consistency with the user interfaces of other
products that people use together (for example, Cut, Copy, Paste)
Metaphorical Consistency - consistency of user interface objects
(individual or composite objects) with their metaphorical counterparts
and perhaps the most important category - Consistency with how people
work - the one that is sometimes lost in the push to be internally
When people speak of consistency, they tend to focus on visual and
basic interaction, but neglect the most important issue of consistency
with how people work.
From Todd Zaki Warfel:
Drop the term "consistency" and replace it with "predictability." Predictability is really what you're after. Consistency helps with predictability, but isn't the only factor. Predictability is a collective of consistency, past experience, previous knowledge, and visual distinction.
Predictability is the true goal, consistency is only one part of predictability. And predictability improves usability.
From Joshua Porter:
>Check out epinions and to some extent amazon, and most e-commerce
retail sites...they often put the consistency of the interface above
the needs of the content/context, and the result is a template that
all content is poured into. We don't shop the same for each kind of
product, so why are our interfaces for them all the same?
I would also agree with Jeff that when you talk about consistency vs.
inconsistency then you lose. Talk about usage...what are people trying
to do and what is most important to them at that point? Have the
activity drive the design decisions if possible.
From Michael Andrews:
People aren't consistent, why do we expect interactive software
always to be so?