Be willing to admit your wrong or uncertain about your results. The degree of your willingness to be upfront about the results contributes to transparency of the research. Why do you believe this to be the most challenging rule of critical thinking?
Admitting one is uncertain or wrong about their results is often the most challenging rule of critical thinking because a person's motives may be for selfish gain without regard to the integrity of the task being undertaken. The degree of one's willingness to be upfront and honest about the results undoubtedly contributes to the transparency of the research.