Karplus extended Piaget's theory to college students and adults; Piaget's theory included four stages, and he had documented children's thinking in great detail, finding that most children made the transition from the 3rd stage (concrete operations) to the 4th stage (abstract reasoning) by about 16 years of age. Karplus, however, extended Piaget's methodology to older groups and found that many of these individuals had important gaps in their ability to use abstract reasoning in solving scientific, logical, and mathematical problems. His most famous test of proportional reasoning was the Mr. Tall-Mr. Short problem. Karplus further explored and documented the details of college students’ and adults’ thinking as they confronted the issues involved in this critical intellectual transition, finding that many of the issues and problems that he, Piaget, and others had discovered as critical for younger students were still relevant for older individuals, particularly when they were attempting to solve a problem in a discipline that was new to them.