In the 1960s and 1970s, he categorized the solutions into five levels

 • Level one. Routine design problems solved by methods well known within the specialty. No invention needed. About 32% of the solutions fell into this level.

 • Level two. Minor improvements to an existing system, by methods known within the industry. Usually with some compromise. About 45% of the solutions fell into this level.

 • Level three. Fundamental improvement to an existing system, by methods known outside the industry. Contradictions resolved.About 18% of the solutions fell into this category.  

• Level four. A new generation that uses a new principle to perform the primary functions of the system. Solution found more in science than in technology.About 4% of the solutions fell into this category.  

• Level five. A rare scientific discovery or pioneering invention of essentially a new system. About 1% of the solutions fell into this category.