Example: Consider a scale measuring depression symptoms. The scale might consistently produce the same scores for an individual over time, indicating high reliability. However, if the scale is not properly designed to capture depression symptoms and instead measures anxiety, it would be reliable but not valid. The scale would consistently measure something (anxiety), but it would not be measuring what it is intended to measure (depression).
Reliability is a necessary condition for validity, but it is not sufficient. A reliable instrument provides consistent results, but that does not guarantee that it is accurately measuring the intended construct. To establish the validity of an instrument, additional evidence must be gathered, such as conducting validity studies to assess whether the instrument truly measures what it claims to measure.