Can a direction in time be displayed by spatial signs?

It is common practice to assign left and right with sequential steps in processing. It might be surprising that these simple functions may cause problems, but it shows the relevance of psychological knowledge to usability.

Redo and Undo are basic functions in almost every word processor. It is essential to be able to revisit your last actions, because human actions are error-prone. Simply using a function to go back and forth between actions is saving a lot of time and energy. The Tango and Oxygen icon sets use arrows pointing left or right. They are additionally colored yellow (Undo) and green (Redo). But these icons tend to get mixed up, as our Icon Test shows.

Results

The results with 9.6, resp. 9.4 in both icon sets, prove that icons are fast and reliable accessed (read about the methodology of testing icons).

Table 1: Results of the Icon-test.

Oxygen
Redo
Undo
Tango
Redo
Undo

Looking at the assignment ratio cross table though, the difference between Undo and Redo becomes striking. With about 95% the function Undo is associated with the correct icon and only gets mixed up to 3-4% with the Redo icon. But vice versa the mismatch increases to ~14%. That means the term Redo is associated to a certain degree with the yellow left pointing arrow.

Table 2: Cross-table of mutual associations.

Tango Oxygen
Term
Undo 96% 3% 95% 4%
Redo 14% 85% 13% 86%

Discussion

Introspectively you might remember the short hesitation, when you wonder which icon to press for Redo. But why?

Left and right are known to be error-prone. At least with time pressure many people tend to confuse left and right. Asked for directions their hands point the correct way, but their words don’t. It needs concentration to sort it all out, and this effort can lead to mistakes. This explanation follows the idea of linguistic determination.

On the other hand and with ecological psychology in mind, a simple explanation based on real world behavior should be preferred. The physical event of going forward is found in reading, for instance. One puts a sheet from the right and turns it over to the left. An action that is just opposite to the direction of the forward / backward icons!

Last but not least, the mental model should be taken into account. The functions Undo and Redo can be understood in two different ways. First in terms of an operation stack. All input is added to the first list and moved to a second list by undo (or back with redo). Alternatively, one can assume that redo just repeats the last operation, and undo works as comparative function. These two mental models are not the same. While the stack idea contains nothing but user input, the ‘repeat last operation’ function could be used separately (like apply last formatting in former Microsoft Office via Ctrl+Y).

Conclusion

Left and right, as well as forward and backward are dual aspects of one feature and need differentiation when false access should be avoided. But the common idea to go back with left is not necessarily supported in real world and could possibly be improved. Better metaphors could be found on the basis of natural processing.

Do you have any suggestions for a natural metaphor that represents Undo and Redo and does not rely on a direction?