How is intuitive defined in terms of UI? Are there any studies about what objective and measurable qualities make a user interface intuitive? What design principles, patterns, or approaches are most likely to produce a UI that is considered intuitive?
Good question. Wikipedia lists intuition as "thoughts and preferences that come to mind quickly and without much reflection" - so basically, saying a UI is intuitive is like saying it exhibits several positive attributes: it's memorable, discoverable, easy to learn, familiar, matches expectation, and so forth. But let's not take my word for it. Let's refer to the experts!
Jef Raskin wrote the definitive article about intuitive interfaces in 1994. In it, he inspects several quotes discussing intuitive interfaces or intuition, and remarks on what they implied. "When the tools had been learned, [...] they became intuitive. This is a strong clue as to the meaning of 'intuitive'," he says about the author of a a review he read (and then proceeds to reference Star Trek IV). Later, he concludes that "'intuitive' in [a certain] context is an almost exact synonym of 'familiar'". And finally, he arrives at a definition: "Intuitive = uses readily transferred, existing skills."
Jared Spool also wrote about intuitive interfaces in an article in 2005 called "What makes a design seem 'intuitive'?". One interesting thing he points out is that "interfaces can’t be intuitive, since they are the behavior side of programs and programs can’t intuit anything. When someone is asking for an intuitive interface, what they are really asking for is an interface that they, themselves, can intuit easily." Afterwards, he introduces the concept of the Knowledge Gap, which is the difference between what the user knows and what the user needs to know in order to understand how to use the software:
An intuitive interface, he argues, will bridge that gap. He identifies two separate conditions for intuitive interfaces. In the first case, the knowledge gap doesn't exist because the user already possesses all the knowledge required to use the interface. In the second case, the user doesn't notice the knowledge gap since the software is training them to use it.
Finally, he suggests what is needed to design an intuitive interface: "What do users already know and what do they need to know? To build intuitive interfaces, answering these two questions is critical." For the first, he recommends field studies and for the second, he recommends usability testing.
In psychology, the Exposure effect "is a phenomenon by which people tend to develop a preference for things merely because they are familiar with them". This phenomenon was first researched in 1876 by Gustav Fechner and likely has a large impact on how people perceive interfaces from a familiarity standpoint, which affects how they intuit them.
I'm worried that perhaps your question is flawed. You are looking for a general answer like, "the faceted search pattern is always going to be more intuitive than the search suggestions pattern," but any answer like that would ignore the most crucial factor: audience. Intuition, and therefore intuitiveness, is a human thing, not a design thing. In a sense, therein lies your answer. It helps to think about it like this:
The one thing that has been proven over and over to increase the likelihood of landing in that middle area is data. Understanding your audience allows you to identify the patterns, principles, and approaches that are most likely to match your users' mental model.
There are lots of ways to get this information, but observing your audience using a possible design, and asking them questions about their experience seems to have a very high return for the amount of time invested. If you are earlier in the design process, it can also be very helpful to look at what your audience considers substitutes, and see what patterns and approaches they are currently being exposed to (and identify the problems with the current approach).
Hope that you already are familiar with Don Norman.
I think your question/topic is closely related to Normans Gulf of Evaluation and Gulf of Execution.
We always want narrow gulfs in UI. To accomplish that, we can work with Normans design principles.
I think it all comes down to making UI's that has as low required mental model for the user as possible. This is achieved by using consistency and great affordance throughout the UI, I also think that is what @Nick Bedford's comment was about.
A system design presents two aspects, the controls user needs to operate in order make the device or system function and the the domain user is operating in.
The paper Domain Models for User Interface Design by David Benyon compares different valid techniques for designing the model, but clearly concludes
The objects which users think about and interact with when using the system must correspond to objects which they understand and use
While all designers are rightly concerned with the knowledge gap around the system or device controls, this can be over emphasised. If the domain is not central to the design, then
- the designer would miss out on an easy route for UI discovery
- the users will miss an important cornerstone which they can leverage to close the knowledge gap
A UI that presents the terminology and/or visual display of entities that a user is familiar with in their work environment can reduce the knowledge gap. Thus be more intuitive. Let's take a look at two process automation UI's
This UI uses well known button controls and data display, but needs lots of cognitive mapping to the process floor:
This UI uses a novel touch based system, but needs minimal mapping to the process floor: :
While UI elements matter, presenting the users domain clearly in the system or device terminology and layout in a way that matches the users mental model is key factor in delivering an intuitive UI. This applies to all significant domain concepts, abstract (e.g. "Tax") as well as physical ones.