Independent testing corresponds to an independent team, who involve in testing activities other than developer to avoid author bias and is often more effective at finding defects and failures.
The following list shows the increasing levels of independence for testing:
Testing done by developer himself
Independent testers ceded to the development team
Independent Testing Team within Organization
Independent Testers of different Organization
Outsourced test team members of other organization
The tester sees each defect in a neutral perspective
The tester is totally unbiased
The tester sees what has been built rather than what the developer thought
The tester makes no assumptions regarding quality
Isolation from the development team can sometimes lead to outdated documentation reference.
The independent test execution is normally the last stage and affected by any delays earlier in the process.
Developers might be irresponsible for quality as they might assume that independent testing team is there to find the issues within the system
Independent testing can sometimes act as a hindrance to communication.