Kirkpatrick's four-level model is the most widely used framework for evaluating training programs. It consists of four levels - reaction, learning, behavior, and results. Each successive level builds upon the prior level and provides a more precise measurement of the training program's effectiveness. However, higher levels also require more rigorous and time-consuming analysis. Level 1 assesses participant satisfaction, level 2 assesses knowledge gain, level 3 evaluates behavior change, and level 4 looks at business impact.