However, the burden of mental ill-health is challenging to measure, especially across countries, and efforts to measure the prevalence of mental disorders, and the burden of mental disorders, vary across OECD countries. Countries have not been successful at addressing unmet need partly because it is so difficult to identify and quantify. A starting point to identifying unmet need is household surveys that use a variety of screening instruments to identify individuals at risk of or suffering from mental health problems. These instruments are not standardised, however, and estimates of the prevalence of mental health problems can vary widely within the same population. Such surveys suggest that around 5% of the working-age population have a severe mental disorder and a further 15% have a common mental disorder (OECD, 2012). Some, but not all, OECD countries report having a national survey to measure the prevalence of mental disorders (Table 4.1).