Part Three: The details · Finish
Each part of a product is built at a different time by different people. When the same action behaves differently in each, nothing people learn carries over.
Inconsistency is nobody's plan. An online shop's orders screen was built three years ago and saves when you press a button. The customers screen was built last year and saves as you type. In one list the trash icon deletes at once, and in another it asks first. Each team worked inside its own corner.
The person using the product does not see corners. They see one thing, and they are trying to learn it. Learning means forming a rule: "changes save by themselves," "that icon is safe to press." Each time a rule turns out to be true only on some screens, they pay for it, sometimes with lost work. After a few such surprises they stop forming rules at all. They slow down, check everything, and treat each screen as new.
Consistency has a cost. A newer, better idea sometimes has to wait until it can be applied everywhere, or be carried back through the old screens. Teams resent this, because the old screens are not theirs and the improvement is ready now. But a better behavior in one place and the old one in five others is, for the customer, a sixth thing to remember.
Orders
Ships in 2 daysSaveCustomers
Dana WhitfieldChanges save automatically
Orders
Ships in 2 daysSaved just now
Customers
Dana WhitfieldSaved just now
Therefore:
Decide once how each common action behaves: saving, deleting, closing, going back. Apply the decision everywhere, old screens included, and change a behavior everywhere or not at all.
In practice. Pick one action, such as deleting. Find every place in the product where it can be done and write down what happens in each. Choose the best behavior and make a list of the screens that differ. An inconsistency that is on a list has a chance of being fixed. One that lives in a customer's head does not.
Goes with. This is A FEW CHOICES, MADE ONCE (66) applied to behavior. Bring old screens into line at the pace of CHANGE SLOWLY WHAT PEOPLE KNOW (56), and leave the smallest differences to the habit described in SOMEBODY LOOKED (85).