According to the Centre for Evidence Based Practice in Oxford UK,

Critical appraisal is the systematic evaluation of clinical research papers in order to establish whether a study:

1. Addresses a clearly focused question?
2. Uses valid methods to address this question?
3. Finds valid results which are important?
4. Finds results which are applicable to my patient or population?

If the study in question does not adequately answer all of these questions, there is no point continuing with it.