Fig. 1From: Employees of the banking sector in Guizhou Province in China: prevalence of migraine, symptoms, disability and occupational risk factorsFlow diagram of the two-stage probability sampling. Abbreviations: PSU, Primary Sampling Units; SSU, Second Sampling Units; ICBC, Industrial and Commercial Bank of China; CCB, China Construction Bank; PPS, Probability Proportional to SizeBack to article page