Besides evidence itself, the second key factor is how evidence is integrated within our reasoning and how we convey our conclusions to their intended recipients. Any good evidence, such as a valid diagnostic test or the effectiveness of a new vaccine demonstrated in an impeccable clinical trial, is useful and beneficial only when applied critically. In other terms, it can be used well or poorly depending on how we introduce it in our reasoning about the health problem under scrutiny. Did we properly assess the risks and benefits of an intervention? Did we properly choose the individuals who would benefit the most from a new treatment? And so on. Critical thinking can be defined as ‘the process of evaluating a claim for the purpose of deciding whether to accept, reject, or perhaps suspend judgment about it… or as … reasonable reflective thinking that is focused on deciding what to believe or do.’ 1 A simpler definition was proposed by Ennis 2,3 : ‘… Reasonable reflective thinking that is focused on deciding what to believe or do ….’ For Harrison, 4 rational thought means ‘… to analyze reasons given for different statements and to determine how these reasons are related in justifying and/or understanding other statements.’

As we will see further in this chapter, critical use of evidence is necessary not only at the patient's bedside, but also equally in research and in health policies and programs. In public health, critical thinking is vital in properly presenting to governmental bodies, decision-makers and other stakeholders, the soundness and relevance of health programs and policies that we propose, implement and evaluate. Critical thinking in medicine is about deciding and conveying well to others what we are doing or what we intend to do, not for our personal intellectual satisfaction, but for the full benefit of the patient or the community. The following topics are worth covering.