What levels of evidence are present in relation to research and practice and why are they important regardless of the method you use?

1)What levels of evidence are present in relation to research and practice and why are they important regardless of the method you use?

2)In addition to the initial question please address the following. This can help you in your participation:
Pick two levels of evidence and develop a table and identify two advantages/disadvantages of each…try and make it relevant to your topic.

3)What factors must be assessed when critically appraising quantitative studies (i.e., validity, reliability, and applicability)? Which is the most important? Why?