JOHANNESBURG (AP) — Once again, there is no winner of a $5 million award for African leadership.

The Mo Ibrahim Foundation announced Tuesday that no former African leader had met the "very high bar" for one of the world's richest prizes in 2016. The Ibrahim Prize has been awarded just four times since it was established in 2006.

Candidates are African heads of state or government who have left office in the past three years, been democratically elected and served their constitutionally mandated term. They must also show "exceptional leadership."

The award honors role models on a continent where many leaders have stayed in power for decades.

Previous winners include former leaders of Namibia, Cape Verde, Botswana and Mozambique. Former South African leader Nelson Mandela was named the inaugural honorary laureate in 2007.