Gross enrolment ratio
Gross Enrollment Ratio (GER) or Gross Enrollment Index (GEI) is a statistical measure used in the education sector and by the UN in its Education Index to determine the number of students enrolled in school at several different grade levels (like elementary, middle school and high school), and examine it to analyze the ratio of the number of students who live in that country to those who qualify for the particular grade level. The United Nations Educational, Scientific and Cultural Organization(UNESCO), describes 'Gross Enrollment Ratio' as the total enrollment within a country "in a specific level of education, regardless of age, expressed as a percentage of the population in the official age group corresponding to this level of education." 
An elementary formula used by most countries to calculate the Gross Enrollment Ratio is that, the country divides the number of individuals who are actually enrolled in schools by the number of children who are of the corresponding school enrollment age. Gross Primary School Enrollment Ratio considers children usually between the age of 6 - 11. Gross Secondary School Enrollment Ratio considers children usually between the age of 12 - 17. Gross Tertiary Education Enrollment Ratio considers the number of young people in the five-year age group following the secondary school leaving age, that is usually 18.  GER is given by the number of students enrolled in educational sector of different levels - 'Primary','secondary' and 'tertiary' level of education by the population of school age children of these levels.
For example, if a nation has 900,000 people enrolled in school in the academic year 2005/06, this number is divided by the total number of school-age individuals. Suppose this number is 1,000,000. This means 90 percent of the people are enrolled; or that 90 percent is the Gross Enrollment Ratio of that nation.
GER = number of actual students enrolled / number of potential students enrolled
The gross enrollment ratio can be greater than 100% as a result of grade repetition and entry at ages younger or older than the typical age at that grade level.
UN Human Development Index use
A combined gross enrollment ratio (CGER), incorporating all three levels of education, is used to calculate the Human Development Index (HDI), an annual gauge of well-being for UN member states. Amongst other measures used in the calculation, the CGER is given one-third weight in assessing the knowledge component, represented by gross enrollment, while the adult literacy rate is assigned two-thirds weight.
- Official UN definition for GER
- How to calculate Gross Enrollment Ratio
- http://hdr.undp.org/hdr2006/pdfs/report/Techinical_notes.pdf page 2