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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 is 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. Techincal 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.
Another form of usability testing is Competative 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.