When students cheat and get a scorer higher than they should have, it is a source of error. The list of kinds of errors goes on and on. This discussion of test characteristics and the various kinds of items is intended to reduce this error. Reliability Reliability is related to the consistency with which a test measures and the confidence that one's obtained score is close to one's true score. Theoretically, an unreliable test cannot be valid, and just because a test is reliable does not guarantee it is valid.
Consistency refers to whether a test has something about it that causes inconsistent results.
For instance, on an essay test when you grade all of your 150 eleventh grade essays, if someone put a copy of the essays of several students who were graded early back into the stack and you regraded the essays without recognizing them, the chances are very high that the regraded essays would receive a different grade the second time they were graded. Reliability is also related to sampling. Tests can only sample the content that is covered in a chapter or unit. So, generally speaking, the longer a test, the more likely it will be providing an adequate sample of behavior. However, common sense tells us that for younger children and where time is short, length may be overridden by the practicality issue, and it should be.