The USPSTF uses an explicit analytic framework to define the scope of the evidence review for each topic it considers. The scope includes specifying the population at risk, the accuracy of screening tests, the efficacy of interventions (treatments) in improving health outcomes, and the harms caused by screening and by the interventions. Before arriving at a recommendation, the USPSTF takes into account the strength of the overall evidence and the balance between the magnitudes of benefits and harms (or the net benefit). When there is adequate evidence to determine the magnitude of the net benefit of delivering a preventive service, the USPSTF assigns 1 of 4 letter grades, A, B, C, or D, which indicate that the net benefit is substantial, moderate, small, or none, respectively. When there is inadequate evidence to determine the net benefit, the USPSTF assigns an I letter grade. The I letter grade simply reflects the lack of adequate evidence to make a recommendation; it is neither a recommendation for nor a recommendation against providing the service on a routine basis. A preventive service could receive an I letter grade for several reasons: Studies may be lacking, existing studies may be of poor quality, or good-quality studies may have conflicting results. Therefore, an I recommendation is a call for additional research that would provide the appropriate evidence base for the USPSTF to make either a positive or negative recommendation.