Editor’s note: Dr. Michael Maddox is Senior Scientist, HumanCentric TechnologiesIn the field of human factors, there are few hard and fast product design rules, but what rules do exist, they can tell you a great deal about certain things.

If you want to know the optimal size for symbols and characters on a display, in order to be legible, it can tell you that.

If you want to know about reasonable navigation structures in a computer application, such as a website, it can help you with that, as well.

It can also tell you that humans are very good at some things and not very good at others.

When you get right down to it, however, a large part of what human factors experts do involves testing to make sure things are as they should be.

In the midst of product design questions, few principles have been developed and proven over the approximate 50-year life of the human factors profession. One of these principles relates to consistency. There are many ways to articulate this principle, but I prefer to put it this way – “If you can’t get it right, at least be consistent.” This phrasing of the principle gives short shrift to using a good, user-centered design process to get it right. Many product designers and businesses, however, fall into the “can’t get it right” category.

One of the nicest characteristics of the consistency principle is that it tends to maximize both usability and efficient use of resources. It’s a win-win situation that is good for users and for business. A couple of examples should demonstrate this principle quite nicely.

Oh, Lord, Won’t You Buy Me–

Until last year, I had never been inside a Mercedes-Benz dealership. As part of the process of replacing my 10-year-old Acura Integra, I drove a bunch of pretty sporty cars. As an afterthought, I tried out a Mercedes C230 Kompressor Sport Coupes. I was very impressed and wound up buying one.

Among the things that impressed me is that every C-model Mercedes has exactly the same configuration of dashboard instruments and controls. In fact, even when you move up the food chain into the E and S models, the dashboard remains fundamentally the same. I commented on this and the salesman told me that this was the Mercedes-Benz philosophy. As a customer moves from model to model, or even year to year, they should feel very comfortable that they know where things are and how they work.

This is not to say that there are no “quirks” in the instrument layout and functionality. There are features that violate usability principles. However, once you learn how things work, that knowledge carries over to every model and every model year. Obviously, Mercedes-Benz wants customers to buy newer, more expensive models. Their consistent use of dash layout and control operation makes it easy for customers to do so.

From the manufacturing, maintenance, and logistics perspectives, the use of interchangeable dashboard controls is preferable to using unique controls and displays for every model vehicle in every model year.

Oui, Zay Are Ze Same

If you think car controls and displays differ greatly among manufacturers, you should see some commercial airplane cockpits. Traditionally, airframe manufacturers re-designed flight decks from scratch for every new model. A Boeing 737 was different than a Boeing 757; a B-757 was different from a B-767, and so on. To further complicate matters, airlines demanded their own unique layout and equipment. So the flight deck on a B-737, purchased by Delta, tends to be different than the identical airplane purchased by United.

This was pretty much the case for all commercial airplanes until Airbus developed their A-300 series airplanes. They made a conscious decision to make the cockpits and handling dynamics of these aircraft as similar as possible. They are, in fact, so consistent in their layout and handling that pilots certified on one aircraft type are granted cross-certification for others. Imagine the training cost savings, and how easy it is to design, procure, and build the flight decks.

Just to be clear, I’m not advocating doing a poor job of product design, quite the opposite. However, as we used to say in the nuclear power business, consistency covers a range of design sins. There are entire books written on this topic. Even if a product or system user interface is weird and violates certain usability rules, users will eventually figure out how to use it. If comparable products have the same design, then all that learning on the part of users will serve a good purpose.

Of course, the best way to proceed with product design is to make the user interface good and consistent.

Michael Maddox received his doctorate in industrial engineering and human factors from Virginia Tech. He is a Senior Scientist for HumanCentric Technologies (www.humancentric.com ), with a focus on risk analysis and human error reduction, management and education. Mike can be reached at 919-481-0565 or mmaddox@humancentrictech.com.