The Grading of Recommendations Assessment, Development and Evaluation (GRADE) working group has developed a common, sensible and transparent approach to grading quality (or certainty) of evidence and strength of recommendations. Many international organizations have provided input into the development of the GRADE approach which is now considered the standard in guideline development.
"The USPSTF is an independent panel of non-Federal experts that makes recommendations on clinical preventive services, such as screenings, counseling, and preventive medications, to primary care clinicians. USPSTF recommendations apply to people with no signs or symptoms of the disease, and are based on a rigorous, systematic review of peer-reviewed evidence." -- webpage 11/2/17