Reliability Reliability Psychological tests are measurement instruments and are similar to rulers, altimeters, and thermometers. A psychological test measures how much the test taker has of a certain quality that the test is measuring. For example a driving test measures how well a person drives. An interest inventory measures things you are interested in.
Consistency is the most important aspect of a measurement instrument especially psychological test.
Often in psychology consistency is referred to as reliability. For example each time you use a ruler to measure an object you get approximately the same measurement. This is why a ruler is consistent or reliable. From time to time measurement errors can occur causing the reliability of the measurement to be off slightly. Not all of these methods are used for all tests.