Reliability refers to the consistency of the scores obtained — how consistent they are for each individual from one administration of an instrument to another and from one set of items to another. We already gave the formula for computing the reliability of a test: for internal consistency; for instance, we could use the split-half method or the Kuder-Richardson formulae (KR-20 or KR-21)
Reliability and validity are related concepts. If an instrument is unreliable, it cannot yet valid outcomes. As reliability improves, validity may improve (or it may not). However, if an instrument is shown scientifically to be valid then it is almost certain that it is also reliable.
The following table is a standard followed almost universally in educational tests and measurement.