In the 1990s, P. Davis et al. published "New Zealand Socioeconomic Index of Occupational Status", known as NZSEI. It was based on a 'returns to human capital' model of the stratification process and originally used data from the 1991 New Zealand census (n=1,051,926) to generate scores for 97 occupational groups. It was later updated using 2006 Census data. NZSEI is a linear scale of ranked occupation, produced using an algorithm involving age, income and education, and aggregated to six discrete groupings (called "Socio-economic Status", SES) to enable comparison with E-I and ISEI.