Usability Rule #1: Consistency is More Important Than Correctness

One of my favorite usability rules is "consistency is more important than correctness." Just to be clear, this is not intended to diminish the importance of correctness, it simply means that if you can't be correct everywhere, you should at least be consistent. Consistency can help lower training costs and reduce mistakes.

As an example of this rule, let's take a complex application with many forms. Dates are displayed on many of these forms using a hard coded format of Month/Day/Year. You have been tasked to create a new form. As a good, conscientious developer, you know that dates should be formatted differently based on the culture of the user and that there is a built in function for formatting the date that actually requires less effort on your part than formatting the date yourself.

What should you do?

1. Format the date correctly using the built-in culture sensitive formatting.
2. Use the same hard coded format as every other place in the application.

Of course, the best answer is C, fix all the other places in the application where the date is formatted incorrectly. Unfortunately, that's not an option you. You don't have access to the other source code, there is business logic relying on the date format, nobody has reported it as a problem before, there is not enough time to fix it, we don't have the testing resources to test in different cultures, etc. (I'm sure you have heard plenty of reasons why people don't want to fix incorrect code).

The second best option is B, format the date consistently with all the other dates in the system. If all the dates are formatted wrong and the user is forced to use the software, they will eventually learn how the date is formatted and will expect the date to always be formatted that way. If their culture reverses the month and day (Day/Month/Year) the user may not realize that you are formatting it correctly and read the date incorrectly possibly causing costly mistakes (a customer doesn't get charged for 6 months, the company's domain name expires, a patient doesn't get scheduled for treatment soon enough and dies, etc.).

Of course, data formatting is not the only thing that should be consistent. Here is a list of common things to look for...

• Formatting of data - date, time, elapsed time, currency, numbers, etc
• Theme (colors and fonts) 
• Unit-of-measure - Be consistent where you can and always make sure the unit of measure is clearly marked.
• Captions - If a field is called XXX on one screen, it should always be called that. Same goes with commands (such as buttons, links, menu items, etc) and any other non-data text displayed to the user. This applies to icons as well.
• Shortcuts - Common commands should have the same shortcut across the entire application.
• Messages - Decide on a common style and use it consistently (for example, friendly or professional)
• Margins - Space your controls consistently. Different controls may require different margins, decide up front what those should be and stick with it.
• Alignment - Are captions aligned left? right? top? bottom? Are numbers aligned to the left? right?
• Layout - How do you group controls (group boxes, horizontal rules, tabs)? Where do you place commands that are related to data fields?
• General flow - How does a user open a record? Save it? Delete it? Create a new one? Access related data? View messages? How are errors handled?
• Consistent with other applications the user may be familiar with - If a user is already familiar with another application, you can leverage their skill from that application in your own.