Problem: The researchers were experimenting on the effects of choice of reinforcers on severe problem behavior. One problem behavior that was being measured was self-injury in the form of the client hitting their elbows on the table. When the principal investigator examined the data from two independent observers there were high levels of disagreement. Upon questioning the observers, the principal investigator learned that one observer would score self-injury only when the child hit the table with their elbow with excessive force from a distance of over 6 inches away. The other observer would score self-injury also if the child lightly tapped the table with their elbow. The major threat to data evaluation validity in this program would be called?