The threat to instrumentation refers to the threat to validity that comes from the instrument used when collecting data. In this case, the instrument can be the one used to code the behavior and it can also be the undergraduate research assistants. I suggest that, because there is more than one undergraduate research assistant participating in the study, Dr. LaGuardia should have a meeting before with them to be clear about the behaviors they will be observing, the instrument provided for them should also be clear and should leave no place for subjectivity when coding the behaviors observed.
