Standard diagnostic criteria for epidemiological studies of psoriasis are currently lacking. In their absence, clinical examination and diagnosis of psoriasis by dermatologists provides the gold standard to underpin epidemiological research in psoriasis. The methods of data collection used most frequently include cross-sectional studies, case-control studies, cohort studies, and surveys of hospital and private practice attendance. Estimates of psoriasis prevalence made in cross-sectional studies employing clinical examinations as the survey instrument have ranged from 0.3 to 2.5%. Psoriasis incidence has been estimated at 60.4 per 100,000 person years in one cohort study. Several factors have been identified as being associated either with causation of psoriasis or with triggering exacerbations or remissions, including genetic determinants, racial and regional variation, injury and infection, cigarette smoking, alcohol, diet and other diseases.