Do’s and Don’ts of Usability Testing

I recommend to read article "Do’s and Don’ts of Usability Testing" (By Demetrius Madrigal and Bryan McClain) before you start usability testing.

Briefly:
+ Get involved and observe usability test sessions.
- Jump to conclusions based on a couple of test sessions.
+ Provide direction to your user research team.
- Dictate to your user research team.
+ Require a screener that includes user behavior.
- Handcuff your recruiter with unnecessary requirements.
+ Get video clips of the test sessions.
- Expect to watch a complete video of a usability study.
+ Perform iterative usability testing.
- Leave target user groups out of your studies.
+ Minimize your impact on test sessions.
avoid: Hiding yourself from participants.
+ Get to know your user research team.
avoid: Using functional prototypes that are overly buggy or unstable.
+ Incorporate experience testing.

When you had even at ones "why?" in your head, read the article.