I have a question which I have been thinking about. Could a scale used to measure relapse behaviour of a drug user be reliable but not valid, and why or why not?

You are most welcome and please feel free to answer the question.

Thanks a ton!

Yes, a scale used to measure relapse behaviour of a drug user could be reliable but not valid if it is consistently measuring the wrong construct. For example, if the scale is measuring drug use rather than relapse behaviour, it may be reliable in consistently measuring drug use but not valid in measuring relapse behaviour.