This is a stub. Please help.
There are various ways to test usability some of which overlap with straighforward bug finding, accessibility testing and internationalisation work. Usability inevitably involves certain tradeoffs but for Inkscape it is important to make basic tasks easy and difficult tasks possible. Inkscape is a drawing tool designed for the mass market. Artists should be considered the primary audience. Technical drawing is a secondary audience and there are many things we will want to make possible but Inkscape is not intended for Computer Aided Design (CAD) and although we should be able to make many of these things possible it would be counter productive to overwhelm our target audience and this complexity must be balanced. The first most important lesson is if Inkscape is difficult to understand it is *not your fault* and if there is any way we can improve things we are interested in exploring them. However we must also look at the big picture, we must avoid microoptimisations (scripting and automation tools will inevitably be needed at some point).
On the testing mailing list AlanHorkan outlines some task based Usability tests which any user could try out for themselves. The tasks are generic and their general purpose is to see if the basic behaviour of the tools is straight forward enough for relatively simple tasks. http://sourceforge.net/mailarchive/message.php?msg_id=12667354
Another form of usability testing is Competitive Analysis. There are valuable lessons we can learn from OtherProjects particularly commercial products such as Adobe Illustrator and Macromedia Freehand which currently dominate the graphic design market.
There are various forms of usability testing, this is only one.
What I have attempted to do here is outline a task you might ask an ordinary user to do. I hope to put this somewhere in the Wiki where it can be refined and others can try it out for themselves. Testers already familiar with Inkscape could also do the test but intimate knowledge of how Inkscape works will obviously skew the results so a high level of self awareness is needed. Talking aloud while doing the test and attempting to vocalise what you are thinking can be very helpful (but a little awkward and embarassing sometimes). The introduction is supposed to help put users in the right frame of mind. The task is intended to be as simple and unambiguous as possible and should be abstract enough to apply to any vector graphics program.
Inkscape Usability Testing
The Inkscape developers understand Inkscape is not perfect. If a task seems difficult or awkward it is not your fault. If it is not immediately obvious what you are meant to do then perhaps we can improve it. Please make a note of anything you find difficult to understand. Find any problems or inefficiencies and try to describe them as best you can. Do not worry too much about suggesting ways to fix problems, it is up to the developer to step back and look at the bigger picture and see how things might best be improved. Even if you are able to figure it out ask yourself if could it be better?
These are the kinds of questions you need to ask to help improve the usability of Inkscape.
Draw a picture of a House.
Use at least one Ellipse Include a linear gradient in your drawing. Inlcude the text "Home Sweet Home"
Save your drawing with the name house-01.svg
[Notes: might break this task up into stages, suggesting different modifications at each stage to encourage testers to discover particular tools and save different versions of the files. Perhaps also include saving the final version of the file so that it can be put on a web page]
Draw a picture of a face.
Gnome Human Interface Guidelines http://developer.gnome.org/projects/gup/hig/
Gnome User Interface Review Checklist http://developer.gnome.org/projects/gup/ui-review/checklist.html