A "valid" measurement is an indication that does not mislead the observer. In other words, if an observer makes a decision based on the observation from a valid measurement, the decision is appropriate. The observer is not mislead by the data.

From this definition several points can be made:

The indicator of a valid measurement is the appropriateness of decisions based on the data from the measurement. This implies a reasonable level of expertise and judgment in the observer.

A valid measurement may not be particularly accurate. But if a decision is to be based on how an observed situation compares with a standard, then a valid measurement will have sufficient accuracy to meet the user's need.

A valid measurement may not be particularly precise. But if a decision is to be based on how one observed situation differs from another, then a valid measurement will have sufficient precision to meet the user's need.