The importance of clearly formulating a problem statement consisting of a premise and a question may be overlooked. A comprehensible formulation of the premise facilitates the understanding by readers and respondents, whereas unclear formulations lead to higher variance in the answers. In our survey, we ask participants to rate the comprehensibility and clarity of four Extensional Reasoning premises. In the test condition, participants work through a set of 29 Extensional Reasoning problems to familiarize themselves with the material before being asked to rate the premises for comprehensibility. In the control condition, participants conduct the rating with no prior exposure. Familiarization occurs on the more comprehensible premises, whereas unclear premises are rated as incomprehensible in both conditions. Incomprehensible premises can baffle interpretive processes shaping the mental representation. The consequence of using difficult to comprehend premises is that experimental results are unreliable and should be interpreted with care.