Answer the following question: Extract the answer to the question from the following context. Question: What is one of the tiers besides per capita income and life expectancy? Context: The Human Development Index (HDI) is a composite statistic of life expectancy, education, and income per capita indicators, which are used to rank countries into four tiers of human development. A country scores higher HDI when the life expectancy at birth is longer, the education period is longer, and the income per capita is higher. The HDI was developed by the Pakistani economist Mahbub ul Haq, often framed in terms of whether people are able to "be" and "do" desirable things in their life, and was published by the United Nations Development Programme.
Answer:
education