Factors in deciding whether a formal or informal review is needed are:
> The importance of the document.
> Whether it is a final deliverable.
> Whether it is a source for a number of other documents.
> The author's experience.
> Who the document's consumer is?
> Whether it is a review of a partial document.
[Source:Practical PRINCE2 By Colin Bentley]
Also See:
Technical Review
Importance of Review
Pair Programming Review
Types of Review Process Structures
Difference between Formal & Informal Reviews
Software Design Reviews
Formal Review & Informal Review
Walkthrough and Inspection
Peer Review
Software Management Reviews
Test Case review
Code Review