More recent research has also explored writing development. Myhill concentrated on the development of written language skills in adolescents aged 13 to 15. Myhill discovered that the more mature writer was aware of the shaping of text, and used non-finite clauses, which mirrored Perera's results (1984). Other researchers focused on writing development up until late adolescence, as there has been a limited research in this area. Chrisite and Derewianke recognize that the survey conducted by Perera (1984) is still one of the most significant research studies in the writing development field and believe Perera's study is similar to theirs. Chrisite and Derewianke (2010) again propose four phases of writing development. The researchers believe that the process of writing development does not stop when an individual leaves formal education, and again, the researchers highlight that these phases are flexible in their onset. The first phase focuses on spoken language as the main aid for writing development, and the development then takes its course reaching the fourth phase, which continues beyond formal education.