Poverty persistence can be measured by looking at those individuals whose income is below a fixed threshold (usually a proportion of median disposable income) over a three-year period. This measure is computed on the basis of special tabulations from surveys that follow individuals over time. A number of different definitions of persistent poverty are possible. One is to measure the share of individuals who are always poor over the three years (i.e “the persistent poor”). Others include how many people are poor in two out of the three years (“recurrent poor”) and how many are poor only once over this period (“poor only once”). The income concept used is that of yearly disposable income (i.e. after transfers and payments of income taxes and social security contributions) of households, where each person is attributed the “equivalised” income of the household where he or she lives, based on a commonly used factor to adjust for differences in household size (the squared root elasticity).