Criteria, Constraints, and Confusion
Sorting through constraints, criteria, and specifications
There is much confusion in engineering design regarding the relationship between needs criteria, design specifications, and design constraints. For engineering students, this can be frustrating and lead to miscommunication on reports and presentations—not to mention the implications for design. For practicing engineers, the stakes are higher, hindering design progress.
Here I’ll describe the problem I see with design talk and propose a system for thinking about constraints, criteria, and specifications.
The Problem
NASA
NASA, an engineering powerhouse, has well-defined design process that includes “interdependent, highly iterative and recursive processes resulting in a validated set of requirements and a design solution that satisfies a set of stakeholder expectations.” the process includes four key processes:
developing stakeholder expectations
technical requirements
logical decompositions
design solutions
In its STEM content NASA pushes to students, the first lesson in the “Packing for the Moon” series (targeting grades 5-8) is called “Identifying Criteria and Constraints.” In the educator’s materials, they cite ITEA/STL 8G. These are the “Standards for Technological and Engineering Literacy” published by the International Technology and Engineering Educations Association (ITEEA). In these engineering education standards, the design process is defined below:
Design process—a systematic problem-solving strategy that relies on us of defined criteria and constraints, used to develop possible solutions to a problem or need and to narrow down the possible solutions to one final choice.
Notice the explicit use of “criteria” and “constraint.” In fact, both of these are also defined by the ITEA/STL 8G standard:
Criterion/Criteria—A desired specification (element or feature) of a product or system.
Constraint—A limit to the design process. Constraints may be such things as time, costs, space, materials, and human capabilities.
So, criteria are related to what is desired of the system produced, and constraints limit the design process itself. Specifications are only the resulting features or elements of a system; specifications may have been incorporated into a design by desire (criterion) or by an artifact of limitations in the design process (constraint).
ABET
ABET is another respected engineering organization. ABET is an American organization that accredits engineering programs around the world and is highly desired to bring credibility to a degree program. In ABET’s definition of engineering design, we read this:
Engineering Design is a process of devising a system, component, or process to meet desired needs and specifications within constraints.
While definitions of needs, specifications, and constraints weren’t immediately available, one can easily see that the ABET framework is not exactly compatible with the ITEEA framework.
ABET introduces “needs,” which have some apparent difference from “specifications.” Both needs and specifications may be desired and may be met by the final system.
Regarding constraints, ABET further states:
For illustrative purposes only, examples of possible constraints include accessibility, aesthetics, codes, constructability, cost, ergonomics, extensibility, functionality, interoperability, legal considerations, maintainability, manufacturability, marketability, policy, regulations, schedule, standards, sustainability, or usability.
The Same… But Different
As far as similarities are concerned, both ABET and ITEEA seem to consider specifications to be the same: desired features of the final system.
But the differences are illustrative of the point:
Unlike ITEEA, ABET includes “needs,” which seem to be those things that must be true of the final system. In that sense, it feels odd to modify the word “need” with “desired.” Needs, by common understanding, are surely desired, but more than that, they are necessary. We might take the idea of a
Constraints for ITEEA are those things that limit the design process. For ABET, however, example constraints include the design process (e.g., schedule) as well as the system itself (e.g., ergonomics, maintainability, usability, etc.).
A Proposal
Definitions
The process of innovation and design is well-described by the Stanford Byers Center for Biodesign in the 2015 book Biodesign: The Process of Innovating Medical Technologies, which I use for my course on Medical Device Innovation and Entrepreneurship. The authors describe the steps of innovation as:
Observation
Problem Identification
Need Identification
Solution
The book puts much attention on writing a strong need statement, which incorporates are called “needs criteria.” In the context of engineering, which has a strong history of design process thinking, to avoid some language like “specifications” and “constrains” was striking.
In the textbook, needs criteria are presented as explaining the need statement. These are things that must be true of the solution that meets the need and solves the problem.
For the sake of my clarity on this issue, I have landed on the following resolution, which makes use of each of the words at play in their more expansive definitions.
Needs criteria are those things that help explain the need statement.
There are two types of needs criteria: design specifications and design constraints.
Design specifications are the things you want to be true of the final system and the design process. “The solution should…”
Design constraints are the things that must be true of the final system and the design process. “The solution must…”
An Example
Below is an example of how this might look for a design process.
1. Need Statement
A way to early-detect Alzheimer’s disease (AD) in the retina using Optical Coherence Tomography
2. Needs Criteria
2.1. Design Specifications
The solution should detect AD with greater than 99% specificity. [system]
The add-on solution should weigh less than ____ kg [system]
The solution should be developed with at least 25 considered design alternatives. [process]
2.2. Design Constraints
The solution must use OCT equipment already existing in ophthalmology clinics plus add-on components [system]
The solution must not expose a patient’s eye to more than the maximum permissible exposure of ____ (check ANSI) [system]
The solution must be developed within 1 year [process]
The solution must be developed with appropriate risk management strategies (check ISO 14971) [process]