This research looked at the link between truancy and pupil performance in examinations. The authors did not look at approaches to tackling truancy but only at the statistical relationship. The majority of the studies were American, emanating from psychiatry and using scientific quantitative approaches to research and a medical model of mental health. Since neither interpretation was verified, this outcome should not be assumed an indication of positive change: the fact that Hensley does so may be an indication of bias towards the integrated model. There is very little research into the effectiveness of parental prosecution, but one factor that was highlighted in both pieces of research was the difference between primary and secondary school pupils. There are several important reasons why phylogenies play an important role in the study of evolution. Lamkin (1990) defines organic agriculture as "a production system which avoids or largely excludes the use of synthetically compounded fertilisers, pesticides, growth regulators and livestock feed additives".