NHS Recommendations for Color Coding Devices
Recently the FDA has been making an evener strong push to have medical device manufacturer's design their products more effectively with regards to human factors. By requiring manufactuers to addess ergonomics, usability and user neer needs in their product development process, we can expect a reduction in errors and improvements in ease of learning and use.
Towards that end, the FDA recognized HE75 as a standard human factors guideline in 2010. This guideline for the design of medical devices provides over 500 pages of human factors principles and recommendations on anthropometrics, user interface design and more.
HE75, like many other consolidated human factors references, is an ambitious effort to consolidate a diverse body of knowledge into a usable resource. I admire it's scope, but it sometimes fall short on clarity. More importantly, the document violates a basic tenet of human factors design - know your user. Much of the content is very technical and ambiguous, which makes it a challenging tool for someone looking for a direct answer. I present a couple of representative examples for consideration.
Sizing a Font
Suppose I am designing a medical device and wish to find out how large to make the type on the screen so that it is readable. This is actually a non-trivial question because readability depends on a number of variables including distance, illumination, and angle, not to mention the acuity of the viewer. But all I want to know is what font size to use. Fortunately, HE75 provides a reference table (see below) of recommended font sizes for various reading distances. The table provides recommendations for six different conditions, including:
- Minimum threshold
- Adequate (lower bound)
- Adequate (upper bound)
- Preferred (lower bound)
- Preferred (upper bound)
but no rationale or definition for what each of these categories mean. Should I assume that if I meet the minimum threshold then I will have created a sufficient design? If I have the space, should I go to adequate, or preferred? Is upper bound for adequate better than lower bound for preferred?
Making matters more complicated, there is another section on type display (hundreds of pages later into the guidelines), that includes this statement:
The minimum character height should be 16 minutes of visual angle.
It would have been great to put that closer to original table, but at least I now know that I should use a minimum visual angle of 16 minutes, which is actually equivalent to "Adequate (lower bound)" in the table, and not "Minimum threshold" - got that? Accordingly, at a reading distance of 36 inches away, the font size should be 12.
Or should it? If I continue reading, I also learn that:
the recommended character size of important but noncritical information is 1/300th of the viewing distance, so a key parameter value on a patient monitor viewed from 3 feet away should be at least 1/8-inch high (a 9-point font: 1 point equals 1/72.27 inch).
So now 9 points is okay from 3 feet (36 inches) even though I previously understood that 12 points would be the miniumum (based on 16 minutes of visual angle from 36 inches)? I have a PhD in human factors and find this challenging - am I missing the subtleties? For such a fundamental question about font size, the guideline is confusing at best, and hypocritical at worst.
Viewing an Alarm
A second example is to find out the visibility requirements of an alarm indicator. Alarms are a critical design feature for obvious reasons, and even have their own standard - HE75 provides a dedicated chapter to alarm guidelines in addition to referencing IEC 60601-1-8.
One question for a device designer relates to placement of an alarm indicator - does it need to be seen from just the front or from all around the device? HE75 provides the following guidance:
IEC 60601-1-8 mandates that point-source visual alarm signals be visible at a viewing distance of 4 meters and from 30 inches off the perpendicular axis.
This is a specific and clear recommendation. So when I went to IEC 60601-1-8 to view the original reference in more detail, I was surprised that there is no mention of "30 inches", but there is a reference to 30 degrees regarding alarm visibility:
the viewpoint is at the OPERATOR'S POSITION or at any point within the base of a cone subtended by an angle of 30° to the axis horizontal to or normal to the centre of the plane of display of the monitoring display or visual indication
Disregarding whether either of these statements is understandable to a lay person in written terms (no diagram or visualization is provided), are these two guidelines equivalent? Is "30 inches" a mistype of "30 degrees" when going from one reference source to the other? If anyone knows, please let me know.
A User-Friendly Alternative
Given the challenges in understanding and interpreting the FDA's recommended documentation, it is not surprising that many medical device manufacturers struggle for guidance and hire consultants to sheperhed them through the human factors process. And it's all the more impressive and refreshing to see a guidelines document that is user-centered and well designed.
The United Kingdom's NHS (National Health Service) has been putting out a series of patient safety guides that are effective for two reasons. First, they focus on a narrow set of topics that are device specific, such as intravenous connectors or urinary catheters. Second, and more importantly, the documents are presented in a clear, direct and visual manner. Specific issues are succinctly described and illustrated, followed by succinctly described and illustrated recommendations. Here's how text size on medication packaging is addressed (note that the recommendation for 12 point size is clearly communicated):
One might make the case that these NHS guidelines are over-simplifications; that they generalize too much and don't accomodate for the variability of real-world conditions. This may be true in some cases, but I would counter that the simplification is much more in the communication, not the content.
Ultimately, the NHS documents come from a customer-facing design approach, while the U.S. documents are derived from engineering, technical standards. Ironically, it is when you provide this greater level of detail and complexity, that you need to provide visualizations to help communicate information more effectively. The cliches abound, but when it comes to design and usability, the FDA (and AAMI) needs to change from a perspective of "do what I say, not what I do", to one of "show me, don't tell me."
