Policy makers and researchers require information on how indicators change over time. An analysis of the impact of reforms on the education system, would be an example, where policy makers would seek to measure changes in the targeted area to gauge the effectiveness of their policies. In the early 1960s, for example, most OECD countries implemented education reforms to facilitate access to tertiary education, mainly through financial help. One indicator of the impact of these reforms would be to calculate the percentage of the population with a tertiary qualification for several years to show how this has evolved. Computing this trend indicator is a straightforward statistical manipulation, since the measure (i.e. whether or not an individual has completed tertiary education) is objective and available at the population level, in most cases. Nevertheless, such measures can be slightly biased by, for example, differing levels of immigration over a period of time, student exchange programmes, and so on.