Explanation:

Donald Kirkpatrick, former Professor Emeritus at the University of Wisconsin, first published his model in 1959. He updated it in 1975, and again in 1993, when he published his best-known work, "Evaluating Training Programs." Each successive level of the model represents a more precise measure of the effectiveness of a training program. It was developed further by Donald and his son, James; and then by James and his wife, Wendy Kayser Kirkpatrick.

In 2016, James and Wendy revised and clarified the original theory, and introduced the "New World Kirkpatrick Model" in their book, "Four Levels of Training Evaluation." One of the main additions is an emphasis on the importance of making training relevant to people's everyday jobs.