Microsoft founder and billionaire philanthropist Bill Gates tweeted out Thursday the following chart, which he called "the most beautiful" in the world.

The chart shows that worldwide child deaths, defined as the death of a child less than five years old, were cut by half between 1990 and 2015 -- from 12.1 million to 5.8 million. On the Bill & Melinda Gates foundation website, Gates added his "favorite number" as well -- 122 million. That's the total number of child lives saved since 1990, Gates said.  

The data is from the Institute for Health Metrics and Evaluation (IHME), an independent research organization that develops global health data for policymakers. According to IHME, significant reductions in child mortality are the result of maternal education and income growth in the world's poorest places. Technological advances, such as vaccines and new drug treatments, have also helped reduce child deaths, IHME said

A reduction in child deaths was one of the U.N.'s Millennium Goals, a series of eight goals related to eradicating extreme poverty. Set in 2000, the U.N. wanted to see a two-thirds reduction in the 1990 under-five mortality rate by 2015. So although the reduction Gates celebrated in his tweet are impressive, it falls short of the U.N.'s target.

Millions of children continue to die every year from preventable diseases. According to UNICEF, the leading causes of death for children under five years of age are pneumonia, prematurity, birth asphyxia, diarrhea and malaria, and nearly 50 percent of child deaths are linked to malnutrition. In 2013, half of under-five deaths occurred in China, the Democratic Republic of the Congo, India, Nigeria and Pakistan, UNICEF said. 

IHME was launched in 2007 with support from the Bill & Melinda Gates Foundation, which announced in January it was partnering with the University of Washington to donate $279 million to the institute over the next decade.