 Among the criticisms of the HDI is the complaint that it is a mixture of stock measures (life expectancy at birth and literacy rate) and a flow measure (GDP per capita for a given year).To overcome this criticism, Hou, Walsh, and Zhang (2015) proposed a new index called HDIF (Human Development Index Flow), in which they replaced life expectancy at birth by the under-five mortality rate (for a given year), and they also replaced the literacy rate by the gross primary school enrollment ratio for a given year).They calculated both the HDI and the HDIF for many countries and found that "the HDIF and the HDI tend to converge for wealthy countries and diverge for poor countries, especially those with low HDI rankings".