An MD&DI June 1997 Column
One of the bigger challenges in medical device design is minimizing human error. Such error manifests itself in the form of actuation of the wrong controls, improper settings, or delayed responses to critical events.
Figure 1. Anthropometric data can be plotted to provide dimensional information for product design.
More than 95% of all medical devices involve the dexterous use of hands. This article illustrates the use of usability studies for testing and evaluating hand/product interaction. Usability studies measure consumers' performance and their perception of their performance when using a product, a new product concept, or design models. A good usability study tells how consumers perceive a product's quality, aesthetics, performance, and value. It also provides quantitative human performance measures of a design's effectiveness.
The next section outlines several research techniques for designing a good usability study. These techniques enable a designer to account for variations in hand size, grasping strategies and grip architectures, tactile and kinesthetic qualities, control layout and design, and how users perceive these factors.
VARIATIONS IN HANDS
There is a 13.9-in. difference in reach between a 5th-percentile Asian woman and a 95th-percentile African American man. Such anthropometric data have been aggregated into a few publicly available databases compiled by organizations such as NASA. These databases have also been incorporated into software packages for manipulating and analyzing body parts in their realistic positions. The databases typically provide data on the variations in physical dimensions as a function of gender, race, and, to a lesser extent, age. A product's success in accommodating the range of users' hand sizes relies on selecting appropriate dimensions and applying them to the design.
Reach Zones. Identifying static dimensional characteristics is the first step in determining an appropriate design for a device that involves a human interaction. Such data can then be plotted as data points in a dynamic context (see Figure 1). Software programs such as Mannequin (Humancad, Melville, NY) and Jack (Center for Human Modeling and Simulation, Philadelphia) enable designers to determine the best digital reach zone.
Grasping Strategies. Grip architectures refer to the number of fingers, the degree of involvement, and the configuration used to grasp and manipulate a tool or control. A long thin grip field typically elicits a fingertip grip, whereas more of the palmar surfaces of the digits are used for larger grips. If the action requires dexterous control, as in the use of a surgical instrument, then the diameter, along with several other factors such as grip material, texture, and topological detailing, will directly affect user performance. Using anthropometric software, designers can identify optimal grip architecture and grasping strategies.
Tactile and Kinesthetic Feedback. Touch can vary as a result of aging or skin surface qualities, such as gloved or callused hands. This factor should be taken into account in designing the type and form of feedback provided to users.
Control Design. Selection of a particular control should be based on the feature or value being manipulated, the number of levels in a discrete control, and the total number of controls being manipulated by the hand.
Perception and Action. Visual information is sometimes referred to as a product's design language or product semantics. The designer can articulate the design of the product to be more or less intuitive. Using software to manipulate color, form, scale, organization, and texture can each contribute to the explicitness of use and operation.
These factors exemplify the design issues that product developers need to consider to optimize hand/device interaction. Systematically evaluating design and ergonomic requirements using anthropometric data will have a direct and tangible effect on a product's success in the form of fewer errors, better efficiency, and improved functionality. Usability studies provide the basis for product planning, design, and marketing.
Bryce G. Rutter is principal of Metaphase Design Group (St. Louis).