Research by Thomas and Chess used the following nine temperament traits in children based on a classification scheme developed by Dr. Herbert Birch: Thomas, Chess, Birch, Hertzig and Korn found that many babies could be categorized into one of three groups: "easy", "difficult", and "slow-to-warm-up" (Thomas & Chess 1977). Not all children can be placed in one of these groups. Approximately 65% of children fit one of the patterns. Of the 65%, 40% fit the easy pattern, 10% fell into the difficult pattern, and 15% were slow to warm up. Each category has its own strength and weakness and one is not superior to another.