While ergonomics has been established in universities across the world as a distinct science, usability has not shared that disctintion.
In a critical study that software engineer, Rolf Molich conducted in 1998, nine usability teams converged to evaluate Microsoft's Hotmail application, and only a fraction of the errors found were common across the groups. Titled CUE-1, Rolf has since followed that study with four more (CUE-2 through 5).
The interpretation of his findings was that even with the standards and guidelines set out by the pioneers of usability, Jakob Nielsen & Don Norman, and every expert since, usability research doesn't seem to produce consistent results. This is more characteristic of design and personal preference than of reliable science.
To counteract the conclusions of this study and reaffirm usability as a science, major organizations, including the Nielsen/Norman group, have pushed for certification and education that would train professionals to work with the same methods and tools.