A survey of pre/post test data using the Halloun-Hestenes Mechanics Diagnostic test or more recent Force Concept Inventory is reported for 62 introductory physics courses enrolling a total number of students N=6542.  A consistent analysis over diverse student populations in high schools, colleges, and universities is obtained if a rough measure of the average effectiveness of a course in promoting conceptual understanding is taken to be the average normalized gain G.  The latter is defined as the ratio of the actual average gain (%post-%pre) to the maximum possible average gain (100-%pre).  Fourteen "traditional" (T) courses (N=2084) which made little or no use of interactive-engagement (IE) methods achieved an average gain G T-ave=0.23+-0.04 (stad dev).  In sharp contrast, 48 courses (N=4458) which made substantial use of IE methods achieved an average gain GIE=0.48 , almost two standard deviations of GIE above that of the traditional courses.  Results for 30 (N=3259) of the above 62 courses on the problem-solving Mechanics Baseline test of Hestenes-Wells imply that IE strategies enhance problem-solving ability.  The conceptual and problem-solving test results strongly suggest that the classroom use of IE methods can increase mechanics-course effectiveness well beyond that obtained in traditional practice.