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Usability Testing is about evaluating the User Experience (UX).
Peter Morville defines this in the this diagram
Useful
Is your product/content original and does it fulfil a need?
Usable
Product must be easy to use
Desirable
Do people want to use your product?
Findable
Can users locate and navigate your product
Accessible
Is your product accessible to users with impairments?
Credible
Is the information contained trustworthy?
Valuable
Does it increase profits?
You can ask participants to perform specific or routine tasks using your product.
Such as installing the product
Saving a file etc.
Then the important factor is the observation of the user under controlled conditions.
There are multiple measures that can be measured to evaluate the experience of a user during a Usability Test
Successful Task Rate
Critical/Non Critical Errors
Error Free Rate
Task Times
Subjective ratings or Likes/Dislikes and recommendations
Eye Tracking Data
Think Aloud Protocol is when a user will be invited to think out loud whilst using a product. They will say what they are thinking and try to describe their thought processes whilst using a piece of software.
This can provide instant feedback
Direct Observation
There is chances for an observer to assist users
Can be instant after task dialogue
Usability testing can even be conducted at the design stage using wireframe models although it would usually be conducted using a high fidelity wireframe. A prototype involving a wireframe would have hyperlinks and hotspots enabled so that even at an early design stage feedback can be gathered and changes made.
Changes made to layout etc. at this early stage will usually require far less time to to implement as opposed to changes made after Implementation.
Initial Sketch
Low Fidelity Prototype
High Fidelity Prototype