We like to use statistic b/c it is scientific and empirical and places behaviour on a scale. But how does one empirically measure behaviour like one can measure. Arbitrary statistic however they are arbitrary height/weight. 1. statistical criteria: abnormal behaviour deviates from the statistically normal (average/mean) behaviour, what most people do normal standard deviation one standard deviation is 66% (within the normal parameters) How does one measure happy/sad and when it becomes psychological the only way we can measure emotions is self-report not very reliable. 3. developmental norms: statistical milestones , but these are variable (i. e. language, at certain ages one is expected to reach certain physical and psychological milestones. Etiological models of abnormal behaviour: etiology cause and course, however, the two are related biology and environment are intertwined, b/f drugs the most common way to cure depression was psychotherapy. Correlation b/w women who have a flu in their second trimester higher likelihood of having a schizophrenic teen/early adult.