In Greater Cincinnati/Northern Kentucky, the mean age of stroke decreased from 71.2 years in 1993 to 1994 to 69.2 years in 2005 because of an increase in the proportion of stroke in Table 2. Definition of Classes and Levels of Evidence Used in AHA/ASA Recommendations Class I Conditions for which there is evidence for and/or general agreement that the procedure or treatment is useful and effective. Class II Conditions for which there is conflicting evidence and/or a divergence of opinion about the usefulness/ efficacy of a procedure or treatment. Class IIa The weight of evidence or opinion is in favor of the procedure or treatment. Class IIb Usefulness/efficacy is less well established by evidence or opinion. Class III Conditions for which there is evidence and/or general agreement that the procedure or treatment is not useful/ effective and in some cases may be harmful. 