Literacy rate is defined as the total number of literate persons (those who can read and write) in a certain age group expressed as a percentage of that age group. Adult literacy rates are calculated for adults aged 15 and up. Children's literacy rates are calculated for children aged 5 to 14. The United Nations Educational, Scientific and Cultural Organization (UNESCO) defines literacy as "the ability to read and write." The organization estimates that there were 740 million readers and writers worldwide in 2003. It also estimates that 4 billion people, or one in three people on earth, cannot read or write. There are two main ways to estimate adult literacy rates: by asking individuals how they learn to read and write; or by estimating what proportion of the population has achieved specific levels of proficiency. The most commonly used method for measuring childhood literacy rates is the paper-and-pencil test called the International Reading Association (IRA) Readiness Assessment. Researchers administer the IRA to groups of children in different countries to see how well they do on topics such as reading comprehension, writing skills, and mathematics understanding. The IRA has been used in over 70 countries around the world. Adult literacy rates are usually estimated using census data or other large-scale surveys. In many countries, these surveys are conducted every 10 years. The latest survey available for many countries is from 2002.