The difference between usability testing and accessibility assessments

An accessibility assessment tests a product against accessibility guidelines such as the Web Content Accessibility Guidelines (WCAG) 2.1. Usability testing assesses how usable a product is for people with various disabilities who use assistive technologies or who customise content. 

Usability testing helps identify barriers that may go undetected in an accessibility assessment. For example, you may have a data table that's coded correctly but is too complex for people who use a screen reader to navigate. Other issues could be that it's difficult for people who use screen magnification to track where they are, or too complicated for people to understand visually. 

Another example is a form with error messages coded correctly with good contrast. If people browse it with screen magnification or on a mobile, the error message may appear off screen and be lost. 

It's always good to complement an accessibility assessment with usability testing because we can't anticipate every possible barrier people may encounter.