What Role Does Luck (Or Bad Luck) Play On Test Day?

by , Aug 20, 2009

This is the first in a series of common student questions that I'll be answering here on BTG. This is one we hear a lot -- I just had a bad day; I'll take it again soon and make sure to focus next time. If the GMAT were really that susceptible to luck, it wouldn't be a very good yardstick by which to compare individuals with limited data points. So, how big a role does luck play?

There are many factors that play a role in test performance. A test provides a single estimate of your ability based on a sampling of questions at a particular time in a particular context. Simple things, like getting enough sleep each of the two nights before the test, can make a difference. The reliability of the test is a measure of the degree to which the same score would be awarded from taking the test multiple times. The GMAT has very good reliability: on a scale of 0-1, the overall reliability of the GMAT is around 0.92, with the verbal and quantitative section reliabilities at 0.90 and 0.89, respectively.

Because many factors contribute to your performance on any given test day, variation in score from test session to test session is expected. This variation can be quantified, and is called the Standard Error of Measurement (SEM). For the GMAT overall score, it is 29 points. This means that if you achieve two scores within 29 points of each other, your performances are consistent with each other. For the verbal scaled score, the SEM is 2.8 points; for quantitative it is 3.0 points.

So while factors like attentiveness and test day environment can play a part in your score, unless something major happened on test day, don't expect a big change just by taking the test again.