Answer:
construct validity of the effect
Step-by-step explanation:
construct validity in research refers to how much the variables that is used in the research forms a correlation with the things that intended to be measured.
From the case above, we know that we want to research the effect of empathy.
But, people show their empathy in a very wide variety of ways. (for example some of them show it through tears, some of them through direct help, some of them through physical contact, etc). Because of this, we do not have any sure method that applicable to all subjects, which make it impossible to find a correlation between the subject's actions and the amount of emphatic. This will cause research end up lacking in construct validity of the effect.