Here is an insight from the official GMAT® test organizers – GMAC® – The graduate Management Admissions Council.
The GMAC® briefing to the test-preparation community on the transition of the GMAT® exam to new vendors in March – April 2005 had GMAC® Chief Psychometrician Larry Rudner fielded a host of questions about the underpinnings of the computer algorithm that scores the test. He emphasized that candidates must complete every question on the GMAT® exam; skipping a question is not an option, and test takers are best advised to make an educated guess if they are unsure of an answer.
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How I Cracked GMAT?