Illustrating income mobility: new measures

摘要

Over two decades ago, Jenkins and Lambert demonstrated that alternative measures of poverty could be combined using the 'Three Is of Poverty' (TIP) curve; the 'three Is' being the incidence, intensity and inequality of poverty. This paper first takes these TIP curve insights and applies them to income growth based measures of mobility, proposing a new 'Three Is of Mobility' (TIM) curve. Secondly, the paper proposes new illustrative devices for income mobility measured as positional change, or re-ranking, within the income distribution: labelled 're-ranking curves'. Illustrations use data from random samples of New Zealand income taxpayers between 1998 and 2010. These mobility curves represent simple graphical devices that conveniently illustrate the 'three Is' properties of income mobility.