Design Thinking | Employing Design Principles | Defining Ease of Use

What Evangelists of Design Thinking Say

“I would refer the inquirer to Bill Buxton’s great book, Sketching User Experiences, in which he suggests several core elements comprising design thinking,” recommends Leo. “Some of these elements include critique, reflection, abductive thinking—pretending the future is now, and considering the results—and rapid sketching, or physical expression of concepts. Many of us work in the context of engineering groups. Design thinking differs markedly from engineering thinking in at least one dimension: Engineering focuses on reductionism—reducing a design to its minimum requirements to achieve a goal. Design thinking may be the complete opposite—identifying as many alternative designs as possible that satisfy the most requirements and sacrifice perhaps only one or just a few.”

“I first connected with the term design thinking when attending Strategy06, where Roger Martin spoke about ‘Designing in Hostile Territory,’” responds Pabini. “Over the past two decades, my own ideas have been evolving along the same lines as those of key proponents of design thinking, but it was wonderful to comprehend that many leaders in the business community now recognize the value of design thinking. Martin is one of the chief evangelists of design thinking. In his 2009 book The Design of Business: Why Design Thinking Is the Next Competitive Advantage, he provides this description of the ‘three essential components’ of design thinking for business, which he crafted in collaboration with David Kelley and Patrick Whitney:

• ‘deep and holistic user understanding’—UX professionals achieve this understanding through user research and user modeling.
• ‘visualization of new possibilities, prototyping, and refining’—Working with a multidisciplinary product team, UX professionals employ an iterative, user-centered design process like that I described in my recent column, “Design Is a Process, Not a Methodology.”
• ‘the creation of a new activity system to bring the nascent idea to reality and profitable operation’—Though the book fails to elaborate on the explicit meaning of this last point, my understanding from the rest of the book is that this involves both creating an organizational culture that understands and fosters design thinking and successfully applying design thinking to projects whose goals are to develop innovative products or services that create customer value and achieve profitability. Walking the walk, not just talking the talk.

“In his 2008 article ‘Design Thinking,’PDF for Harvard Business Review, Tim Brown, another strong advocate for design thinking, writes about ‘how to make design thinking part of the innovation drill,’ providing these guidelines, which though more detailed, closely parallel the essential components Martin’s book describes:

• ‘Begin at the beginning. Involve design thinkers at the very start of the innovation process, before any direction has been set. Design thinking will help you explore more ideas more quickly than you could otherwise.
• ‘Take a human-centered approach. Along with business and technology considerations, innovation should factor in human behavior, needs, and preferences. Human-centered design thinking—especially when it includes research based on direct observation—will capture unexpected insights and produce innovation that more precisely reflects what consumers want.
• ‘Try early and often. Create an expectation of rapid experimentation and prototyping. …
• ‘Seek outside help. Expand the innovation ecosystem by looking for opportunities to co-create with customers and consumers. …
• ‘Blend big and small projects. Manage a portfolio of innovation that stretches from shorter-term incremental ideas to longer-term revolutionary ones. …
• ‘Budget to the pace of innovation. Design thinking happens quickly, yet the route to market can be unpredictable. Don’t constrain the pace at which you can innovate by relying on cumbersome budgeting cycles. …
• ‘Find talent any way you can. … People with more conventional design backgrounds can push solutions far beyond your expectations. You may even be able to train nondesigners with the right attributes to excel in design-thinking roles.
• ‘Design for the cycle. … Plan assignments so that design thinkers go from inspiration to ideation to implementation. Experiencing the full cycle builds better judgment and creates great long-term benefits for the organization.’

“Another way of looking at the core elements of design thinking is to assess the attributes a design thinker must have,” continues Pabini. “Tim Brown’s article is instructive about this as well. He writes: ‘Here … are some of the characteristics to look for in design thinkers:

• ‘Empathy. They can imagine the world from multiple perspectives—those of colleagues, clients, end users, and customers…. By taking a people first approach, design thinkers can imagine solutions that are inherently desirable and meet explicit or latent needs. Great design thinkers … notice things that others do not and use their insights to inspire innovation.
• ‘Integrative thinking. They not only rely on analytical processes …, but also exhibit the ability to see all of the salient—and sometimes contradictory—aspects of a confounding problem and create novel solutions that go beyond and dramatically improve on existing alternatives. …
• ‘Optimism. They assume that, no matter how challenging the constraints of a given problem, at least one potential solution is better than the existing alternatives.
• ‘Experimentalism. Significant innovations don’t come from incremental tweaks. Design thinkers pose questions and explore constraints in creative ways that proceed in entirely new directions.
• ‘Collaboration. The increasing complexity of products, services, and experiences has replaced the myth of the lone creative genius with the reality of the enthusiastic interdisciplinary collaborator. The best design thinkers don’t simply work alongside other disciplines; many of them have significant experience in more than one….’”

What UXmatters Authors Say on These Topics

“Read Jim Nieters’s ‘Is Your Design Thinking Showing?’” suggests Pabini. “And, for further discussion—in my own words—of some of the ideas I outlined earlier, read my UXmatters articles:

• ‘Sharing Ownership of UX’—Explores working collaboratively with multidisciplinary product teams in depth.
• ‘Specialists Versus Generalists: A False Dichotomy?’—Explains how corporate culture affects an organization’s ability to recognize and hire T-shaped people—another way of saying design thinkers—and describes their attributes.

“Past editions of Ask UXmatters also discuss related ideas:

• ‘Evangelizing UX Across an Entire Organization’—In this column, I elaborate on the topic of my article ‘Sharing Ownership of UX.’
• ‘Generating Ideas’—This column discusses how UX designers generate the myriad ideas a design-thinking organization needs.”

Design Thinking or Design?

“There’s an argument that says the core elements that make up design thinking are the same as those that make up design, and it’s one with which I tend to agree,” replies Steve. “The difference can be said to lie in the fact that design thinking doesn’t really get into the specific expertise one needs to design in a particular medium. The refined, detailed, craft side of design doesn’t play a part in design thinking, where prototypes don’t require the same level of fidelity or materials expertise. So, for me, your question is really ‘What are the core elements that make up design?’ and they are:

• observation
• empathy
• deconstructionism
• multiplicity of ideas
• critique
• synthesis and abductive thinking
• multidisciplinary teams
• sketching
• prototyping
• iteration”

Who Coined the Term Design Thinking?

“While a term’s meaning may evolve and grow, it should remain true to its author’s original intent. Otherwise, such terms would have no clear, shared meaning,” contends Pabini. “Roger Martin didn’t coin the term design thinking, but he certainly has given it currency. Nor did Tim Brown or David Kelley coin it, as many believe. (Why is it that those guys from IDEO seem so often to get the credit for terms they popularize?) Adam Richardson, author of Innovation X, attributes the term to Richard Buchanan, who wrote an article for Design Issues titled ‘Wicked Problems in Design Thinking’ in 1992, but that wasn’t the term’s first use either. As far as I’ve been able to discover, the author of the term design thinking is Peter G. Rowe, who introduced it in his book titled Design Thinking, which was first published way back in 1987. Donald A. Schön’s seminal 1983 book The Reflective Practitioner: How Professionals Think in Action was the source of many of the ideas behind what we now call design thinking.

“Interestingly, the focus of Rowe’s book is design thinking in architecture and urban planning. Thus, like the origin of the use of patterns in design, the conception of design thinking arose from the field of architecture.

“So, how does Rowe define design thinking? While he doesn’t provide an explicit definition in his book, he says, ‘Design appears to be a fundamental means of inquiry by which man realizes and gives shape to ideas…. … Design is a practical form of inquiry insofar as it is concerned with making…. To fashion a generalized portrait of design thinking … I am concerned with the interior situational logic and the decision-making processes of designers in action, as well as with theoretical dimensions that both account for and inform this kind of undertaking. … In the give and take of problem-solving situations in the real world, … we start to see the complex textures of decision making. … There is no such thing as the design process in the restricted sense of an ideal step-by-step technique. Rather, there are many different styles of decision making, each with individual quirks as well as manifestations of common characteristics. Sometimes the unfolding of a design is strongly influenced by constraints derived from the initial setting of the problem…. Certain features [are] characteristic of the problem-solving behavior of … designers, particularly its episodic structure and the fact that the designer relies on presuppositions and hunches … at least as much as on information furnished during orderly confrontation with the constraints found in a given design problem.’

“Rowe concludes, ‘Several observations can be made about the nature of design thinking. … The unfolding of the design process assumes a distinctly episodic structure, which we might characterize as a series of related skirmishes with various aspects of the problem at hand. Usually the results of these investigations cohere into a more singular direction for the design activity, although not necessarily as a linear progression of reasoning.

“‘This episodic structure manifests itself in a number of ways. First, there is the to and fro movement between areas of concern… movement back and forth between exploration of … form and evaluations of program, structure, and other technical issues. Second, there seem to be periods of unfettered speculation, followed by more sober and contemplative episodes during which the designer takes stock of the situation. Third, each episode seems to have a particular orientation that preoccupies the designer. … The organizing principles involved in each episode take on a life of their own, as the designer becomes absorbed in exploring the possibilities that they promise. Here a dialogue between the designer and the situation is evident…. Finally, as the scope of the problem [becomes] more determined and finite for the designer, the episodic character of the process [becomes] less pronounced. During this period a systematic working out of issues and conditions [takes] hold within the framework that had been established. This phenomenon is not at all surprising when we consider the fundamental difference between moments of problem solving when matters are poorly defined and those with clarity and sufficiency of structure.

“‘These episodes … possess an interior logic that seems determined partly by the subject matter at hand and partly by the organizational procedures being used. They also have a consequential connection with one another. Without such logic and closure among episodes, the emergence of design proposals would be difficult to imagine.’

“What’s different today is that we’ve moved beyond the journey of discovery that has led to an understanding of how designers think to an exploration of how we can apply design thinking to different types of problems and how design thinking can help us to drive innovation.”

Employing Design Principles in Our Work

Q: How do design principles factor into your work when you are designing a user interface? At what point do you use them?—from a UXmatters reader

“Different design principles apply at different levels and phases of design,” replies Robert. “Broad principles such as pretending it’s magic or pretending it’s human apply in the very earliest conceptual phases of design. Designers typically employ principles and patterns of good product behavior—such as allowing input where there is output or minimizing navigation—when they’re creating a product’s basic framework, then apply more detailed widget, layout, and visual information principles in the later phase of detailed design.”

“Design principles should guide your decisions throughout a project—not just during the formal design stages—including all of the many little design decisions you make during implementation as well,” recommends Steve. “For design principles to play that role, they should be broad enough not to be prescriptive, but specific enough that you can measure any particular design decision against them to determine whether it aligns well with the design principles you’re following. More to the point, different people should be able to interpret design principles independently, in the same way.”

Tobias agrees, “Design principles yield the greatest effect when you not only deploy them during distinct phases of a project, but they provide the basis for and drive the whole project—that is design thinking. User experience implies that all aspects of a product lifecycle—from ideation to after-deployment support—should revolve around users and their needs. Given that, you need to define your—or your team’s or your company’s—design principles.”

“Design principles have their basis in human cognition, perception, and psychology,” answers Pabini. “Sound design principles provide the foundation for all of the work we do as UX designers. These principles constitute our primary toolkit. Following design principles prevents our making poor design decisions that would impair the usability of the products and services we design. Design principles serve us especially well whenever we encounter unique design problems for which no guidelines, patterns, or frameworks yet exist—when we must think outside the box and come up with innovative design solutions.”

Resources: UX Design Principles

“There are a lot of design principles out there,” offers Tobias. “For example:

• ‘Dieter Rams: Ten Principles for Good Design’
• ‘Google User Experience’
• ‘Microsoft Windows User Experience Design Principles’”

“The earliest books on UX guidelines included chapters on key design principles, which evolved over time,” notes Pabini. “We still follow those principals today when designing user interfaces. You’ll find current expressions of such principles in the documentation of UX guidelines for various computing platforms—in addition to those Tobias has mentioned, including the following:

• Apple Inc. ‘Characteristics of Great Software.’ Apple Human Interface Guidelines. Mac OS X Reference Library, August 20, 2009. Retrieved January 11, 2011.
• —— ‘Human Interface Design Principles.’ Apple Human Interface Guidelines. Mac OS X Reference Library, August 20, 2009. Retrieved January 11, 2011.
• —— ‘Human Interface Principles.’ In iOS Human Interface Guidelines. iOS Reference Library, January 3, 2011. Retrieved January 11, 2011.
• Apple Computer Inc. ‘Philosophy.’ In Apple Human Interface Guidelines: The Apple Desktop Interface, 3rd ed. Cupertino, CA: Addison-Wesley, 1986.
• —— ‘Human Interface Principles.’PDF In Macintosh Human Interface Guidelines. Cupertino, CA: Addison-Wesley, 1995. Retrieved January 11, 2011.
• IBM. ‘Design Principles.’ Design@IBM. Retrieved January 11, 2011.
• —— ‘Design Principles.’PDF Design@IBM. Retrieved January 11, 2011.
• Microsoft. ‘Design Principles and Methodology.’ In Microsoft Windows User Experience: Official Guidelines for User Interface Developers and Designers. Seattle, WA: Microsoft Press, May 3, 2001. Retrieved January 11, 2011.
• —— ‘Design Principles and Methodology.’ In Microsoft Windows User Experience: Official Guidelines for User Interface Developers and Designers. Seattle, WA: Microsoft Press, 1999.
• —— ‘Design Principles and Methodology.’PDF In The Windows Interface Guidelines—A Guide for Designing Software. Seattle, WA: Microsoft Corporation, 1995. Retrieved January 11, 2011.
• Research In Motion Limited. ‘Three Key Principles of BlackBerry Application Design.’ In UI Guidelines - BlackBerry Smartphones, Version 6.0, 2010. Retrieved January 11, 2011.
• Sun Microsystems. ‘Introduction.’ In Java Look and Feel Design Guidelines: Advanced Topics, 2001. Retrieved January 11, 2011.

“The book that covers UX design principles in the greatest depth is: Lidwell, William, Kritina Holden, and Jill Butler. Universal Principles of Design, Revised and Updated: 125 Ways to Enhance Usability, Influence Perception, Increase Appeal, Make Better Design Decisions, and Teach through Design. Beverly, MA: Rockport Publishers, 2003.

“Books that include chapters on UX design principles include the following:

• Cooper, Alan, Robert Reimann, and David Cronin. About Face 3: The Essentials of Interaction Design. Indianapolis, IN: Wiley Publishing, 2007.
  
  • ‘Synthesizing Good Design: Principles and Patterns’
  • ‘Designing Good Behavior’
  • ‘Visual Interface Design’
• Goodwin, Kim. Designing for the Digital Age: How to Create Human-Centered Products and Services. Indianapolis, IN: Wiley Publishing, 2009.
  
  • ‘Principles and Patterns for Framework Design’
  • ‘Principles and Patterns in Design Language’
  • ‘Detailed Design Principles and Patterns’
• Johnson, Jeff. ‘First Principles.’ In GUI Bloopers: Don’ts and Do’s for Software Developers and Web Designers. San Francisco: Morgan Kaufmann, 2000.
• Mayhew, Deborah J. ‘Introduction.’ In Principles and Guidelines in Software User Interface Design. Englewood Cliffs, NJ: Prentice Hall, 1992.
• Mullet, Kevin, and Darrell Sano. Designing Visual Interfaces: Communication Oriented Techniques. Englewood Cliffs, NJ: Prentice Hall, 1994.
  
  • ‘Elegance and Simplicity’
  • ‘Scale, Contrast, and Proportion’
  • ‘Organization and Visual Structure’
  • ‘Module and Program’
  • ‘Image and Representation’
  • ‘So What About Style?’
• Saffer, Dan. ‘Refinement.’ In Designing for Interaction: Creating Innovative Applications and Devices, 2nd ed. Berkeley, CA: New Rider, 2010.
• Shneiderman, Ben, and Catherine Plaisant. ‘Guidelines, Principles, and Theories.’ In Designing the User Interface: Strategies for Effective Human-Computer Interaction, 5th ed. Boston: Addison-Wesley, 2009.

“Here are a few good resources on the Web that outline or discuss some principles of UX design:

• Baekdal, Thomas. “XMLHttpRequest Usability Guidelines.” Baekdal, February 20, 2005. Retrieved January 11, 2011.
• Bell College. ‘Guidelines and Standards.’PDF Human Computer Interaction. Retrieved January 11, 2011.
• Gabriel-Petit, Pabini. ‘First, Do No Harm.’ UXmatters, November 16, 2009. Retrieved January 11, 2011.
• Ginsburg, Suzanne. ‘10 Design Principles for eReader Apps.’ iPhone & iPad UX Reviews, July 26, 2010. Retrieved January 11, 2011.
• Hess, Whitney. ‘So You Wanna Be a User Experience Designer—Step 2: Guiding Principles.’ Pleasure & Pain, November 23, 2009. Retrieved January 11, 2011.
• Lamantia, Joe. ‘Playing Well with Others: Design Principles for Social Augmented Experiences.’ UXmatters, March 8, 2010. Retrieved January 11, 2011.
• Tognazzini, Bruce. ‘A Quiz Designed to Give You Fitts.’ Ask Tog, February 1999. Retrieved January 11, 2011.
• Tognazzini, Bruce. ‘First Principles of Interaction Design.’ Ask Tog, November 14, 2002. Retrieved January 11, 2011.
• Wijnholds, Henk. ‘Design Principles to Choose the Right Ideas.’ Henk Wijnholds, October 26, 2009. Retrieved January 11, 2011.”

Using Design Principles Consciously

“There’s nothing wrong with broad and generic principles,” answers Tobias. “To some extent, principles have to be general in order to be applicable to as many specific instances as possible—for example, for different products, markets, or domains. I believe that there are no right or wrong principles per se. You—or your team or your company—have to think about what principles are right for you. Once you have these, you need to follow them consistently. During a product development lifecycle, this means that, at any stage, you should refer back to your principles and ask yourself whether they are reflected in your design activities and the user experience you’re shaping.”

“The difference between a design or an architecture rests on the UX Architect’s self-conscious choice of design principles,” responds Leo. “Knowing one’s own design principles—being able to articulate them explicitly—is crucial to self-evaluation. They are also crucial to critical review—how can anyone evaluate the success of a specific design solution without understanding the underlying design principles?

“Self-conscious choice of design principles is key. We could unconsciously put elements together and claim we’ve created a design solution, but unless we can articulate why we put those elements together—why they are a better arrangement than some other arrangement or some other elements—we aren’t truly designing, and we definitely are not architecting.

“There are literally thousands of design principles to choose from. Which ones you choose to follow go to the very basis of your design aesthetic. Is there a particular school of thought that attracts you? Then, the likelihood is that the design principles underlying that school resonate with you. The target for your solution—the ecosystem of use—likely has underlying design principles. Those are good candidates when you’re starting your own self-conscious selection of design principles. When the target ecosystem’s principles are fundamentally uninteresting or in complete violation of the UX Architect’s own preferred design principles, the likelihood is that the UX Architect is the wrong match for the problem.

“Design principles are a fundamental pillar of my user experience architecture and influence every design move I make during the day.”

The Ultimate, Theoretical Target for Ease of Use

Q: This is somewhat of an abstract question, but could have a major impact on building in usability at the product definition stage. Assuming user experience is synonymous with ease of use and a design is a clean start, without any legacy issues, how would you define the ultimate, theoretical target for ease of use? This can help to guide our designers to think out of the box and come up with innovative approaches. I am looking for a general definition of an ultimate ease of use target for users who are using a product to achieve a specific goal—but the user is not able to articulate this.—from a UXmatters reader

“Targets for ease of use might be things like ‘The system must respond within 1 second,’” responds Caroline. “Or ‘The user must be able to be productive with the system within 20 minutes.’ Both of these are fine targets—but are by no means universal. On response time: What if the system is crunching terabytes of data? A speedy response could be in the hours or days. On easy-to-learn: What if users will be working all day, every day with large quantities of rapidly changing data—as in a stock-trading system? Then, heavily packed, complex screens will likely be just right—at the expense of making users invest considerable time and effort in learning to use them.”

“The following tools and techniques can help you arrive at an ‘ultimate’ UX target,” suggests Robert. “Through qualitative research to understand target users’ goals—for example, ethnographic techniques that elucidate needs users themselves may not be able to articulate—you can elicit

• life goals—What life aspirations might a product be able to help users achieve? What is motivating users to choose and use this product?
• end goals—What expectations do users have for the outcomes of using a product? What concrete end results or outcomes are they after?
• behavioral goals—What mental models do users have around the tasks they are performing, and how can the product mesh with, enhance, or improve that experience.”

The Real Question

You say you want to find a theoretical target for ease of use to assist your designers in thinking outside the box, but I think you are off track with this question. First of all, it is too vague: there is no one-size-fits-all solution for all design situations. Second, the lack of detail would direct the designer’s focus to what he or she already knows best: his or her skills. As the saying goes, “If you are a hammer, everything looks like a nail.” Given this vague goal of describing the “ultimate target,” your designer would then design based on his or her skills, as opposed to the specifics of the product.

Also, you end your question by stating the condition that “the user is not able to articulate” the ultimate ease-of-use target. Remember, users are not primarily concerned about ease of use; their focus is on meeting their goals. Yes, ease of use is part of a design that enables users to reach their goals, but they may not be specifically aware of that fact. We design a product’s features, but users enjoy the benefits of our design. In the same way that a driver does not need to understand the superior design of a car’s engine to drive successfully, users do not need to understand or designate an ultimate ease-of-use target.

It sounds like you are at the beginning of your project. I wonder if your designers are actually engineers who are making design decisions—like those described in The Inmates Are Running the Asylum. As a classically trained computer scientist turned interaction designer, I can relate if they are. The first thing I would recommend to you is to find a UX design consultant who can help you with the basics of your design. I applaud you for applying design principles at the earliest stages of your project. It is absolutely the best time to start good design work.

“The question is abstract and, it turns out, is likely the wrong question to be asking,” replies Leo. “There is no theoretical target for ease of use. Ease of use is always a practical matter and very specific to your target users. Taking a page from the goal-directed design approach Cooper promotes, you’ll find ease of use is an attribute of something far more important to your target users. Further, you should design for a specific user, not some theoretical user. As a result, the notion of ease of use may become moot—users may not seek ease, but rather efficiency or joy—or just shy of frustration, in the case of game design. It is a bad assumption that user experience is synonymous with ease of use. User experience is... well, the desired user experience.”

“This question intrigues me, but it’s so abstract, I don’t know where to begin,” declares Whitney. “Are you asking about metrics—how to set measurable behavioral targets for ease of use? Or are you asking, how do you understand what ease of use means in a product that is sufficiently novel users won’t have a frame of reference? My short answer to the latter would be that the targets should not be stated in terms of manipulating or interacting with the product, but in achieving users’ broader aims. To put it another way—using Twitter as an example—you might want to know whether

• people find Twitter understandable and useful
• people can discover how to use Twitter on their own
• people who understand Twitter find it easy to interact with
• Twitter’s interaction and interface style meet the needs and fit into the lives of the people who use it

“All of these questions might relate to different ease-of-use targets. And, depending on which of these sorts of questions the product team finds most important, the resulting target would shape the product design.

“Alternatively, you could ask which of my 5Es—efficient, effective, engaging, error tolerant, easy to learn—you are aiming for in the design.”

Applying Your Understanding of Ease of Use to Product Design

Robert recommends “applying these techniques when creating a design:

• Pretend it’s magic. If users could magically achieve their end goals with a product with no work, what would that experience be like? Use this as your starting point, then work back to reality from there.
• Pretend it’s human. If the product were a super-smart, super-helpful human who helps users with their tasks, what would that experience be like? From this starting point, work your way back to reality.”

“It sounds like there may be something implicit in the question, about not being overly prescriptive in defining the details of an innovative product’s design,” notices Whitney. “I agree with that intent: I’d want to see goals defined in terms of broader life impact or outcomes, but might also want to insist that a user interface be very usable, so barriers to interaction wouldn’t interfere with adoption of a novel idea.”

“I wouldn’t define a theoretical target for ease of use,” states Caroline. “I’d develop a target by balancing two equally important factors: what users believe to be an appropriate target and what an organization believes to be an appropriate target. And finally, it’s really important not to make the mistake of thinking only user considerations are important or only organizational considerations are important. Most of the organizations I have worked with have erred on the side of overemphasizing their considerations at the expense of users’ needs, but it’s definitely possible to make the opposite mistake.” 

References

Brown, Tim. “Design Thinking.”PDF Harvard Business Review, June 2008. Retrieved January 9, 2011.

Buxton, Bill. Sketching User Experiences: Getting the Design Right and the Right Design. San Francisco: Morgan Kaufmann, 2007.

Cooper, Alan. The Inmates Are Running The Asylum: Why High Tech Products Drive Us Crazy and How to Restore the Sanity. Indianapolis: Sams, 2004.

Martin, Roger. The Design of Business: Why Design Thinking Is the Next Competitive Advantage. Boston: Harvard Business Press, 2009.

Rowe, Peter G. Design Thinking. London: The MIT Press, 1991.