A test can be reliable without being valid. For example a ruler is a reliable measure. It will always measure a given distance the same way. However, it is not a valid measure of depression, as the outcome of its measurements, no matter how consistent, have no relationship to depression. Although a test can be reliable without being valid, it cannot be valid without being reliable. If a test is inconsistent in its measurements, we cannot say it is measuring what it is intended to measure and, therefore, it is considered invalid.